2018 |
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 | Constantin Krüger, Elina Lisitsin-Baranovsky, Oded Ofer, Pierre-Alexandre Turgeon, Jonathan Vermette, Patrick Ayotte, Gil Alexandrowicz A magnetically focused molecular beam source for deposition of spin-polarised molecular surface layers Article de journal The Journal of Chemical Physics, 149 (16), p. 164201, 2018. @article{Krüger2018, title = {A magnetically focused molecular beam source for deposition of spin-polarised molecular surface layers}, author = {Constantin Krüger and Elina Lisitsin-Baranovsky and Oded Ofer and Pierre-Alexandre Turgeon and Jonathan Vermette and Patrick Ayotte and Gil Alexandrowicz}, doi = {10.1063/1.5048521}, year = {2018}, date = {2018-10-22}, journal = {The Journal of Chemical Physics}, volume = {149}, number = {16}, pages = {164201}, abstract = {Separating molecular spin isomers is a challenging task, with potential applications in various fields ranging from astrochemistry to magnetic resonance imaging. A new promising method for spin-isomer separation is magnetic focusing, a method which was shown to be capable of producing a molecular beam of ortho-water. Here, we present results from a modified magnetic focusing apparatus and show that it can be used to separate the spin isomers of acetylene and methane. From the measured focused profiles of the molecular beams and a numerical simulation analysis, we provide estimations for the spin purity and the significantly improved molecular flux obtained with the new setup. Finally, we discuss the spin-relaxation conditions which will be needed to apply this new source for measuring nuclear magnetic resonance signals of a single surface layer.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Separating molecular spin isomers is a challenging task, with potential applications in various fields ranging from astrochemistry to magnetic resonance imaging. A new promising method for spin-isomer separation is magnetic focusing, a method which was shown to be capable of producing a molecular beam of ortho-water. Here, we present results from a modified magnetic focusing apparatus and show that it can be used to separate the spin isomers of acetylene and methane. From the measured focused profiles of the molecular beams and a numerical simulation analysis, we provide estimations for the spin purity and the significantly improved molecular flux obtained with the new setup. Finally, we discuss the spin-relaxation conditions which will be needed to apply this new source for measuring nuclear magnetic resonance signals of a single surface layer. |
2017 |
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 | Pierre-Alexandre Turgeon, Jonathan Vermette, Gil Alexandrowicz, Yoann Peperstraete, Laurent Philippe, Mathieu Bertin, Jean-Hugues Fillion, Xavier Michaut, Patrick Ayotte Confinement Effects on the Nuclear Spin Isomer Conversion of H2O Article de journal The Journal of Physical Chemistry A, 121 (8), p. 1571-1576, 2017. @article{Turgeon2017, title = {Confinement Effects on the Nuclear Spin Isomer Conversion of H2O}, author = {Pierre-Alexandre Turgeon and Jonathan Vermette and Gil Alexandrowicz and Yoann Peperstraete and Laurent Philippe and Mathieu Bertin and Jean-Hugues Fillion and Xavier Michaut and Patrick Ayotte}, doi = {10.1021/acs.jpca.7b00893}, year = {2017}, date = {2017-02-03}, journal = {The Journal of Physical Chemistry A}, volume = {121}, number = {8}, pages = {1571-1576}, abstract = {The mechanism for interconversion between the nuclear spin isomers (NSI) of H2O remains shrouded in uncertainties. The temperature dependence displayed by NSI interconversion rates for H2O isolated in an argon matrix provides evidence that confinement effects are responsible for the dramatic increase in their kinetics with respect to the gas phase, providing new pathways for o-H2O↔p-H2O conversion in endohedral compounds. This reveals intramolecular aspects of the interconversion mechanism which may improve methodologies for the separation and storage of NSI en route to applications ranging from magnetic resonance spectroscopy and imaging to interpretations of spin temperatures in the interstellar medium.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The mechanism for interconversion between the nuclear spin isomers (NSI) of H2O remains shrouded in uncertainties. The temperature dependence displayed by NSI interconversion rates for H2O isolated in an argon matrix provides evidence that confinement effects are responsible for the dramatic increase in their kinetics with respect to the gas phase, providing new pathways for o-H2O↔p-H2O conversion in endohedral compounds. This reveals intramolecular aspects of the interconversion mechanism which may improve methodologies for the separation and storage of NSI en route to applications ranging from magnetic resonance spectroscopy and imaging to interpretations of spin temperatures in the interstellar medium. |
2016 |
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 | Elina Lisitsin-Baranovsky, Sarah Delage, Oscar Sucre, Oren Ofer, Patrick Ayotte, Gil Alexandrowicz In Situ NMR Measurements of Vapor Deposited Ice Article de journal The Journal of Physical Chemistry C, 120 (44), p. 25445-25450, 2016. @article{Lisitsin-Baranovsky2016, title = {In Situ NMR Measurements of Vapor Deposited Ice}, author = {Elina Lisitsin-Baranovsky and Sarah Delage and Oscar Sucre and Oren Ofer and Patrick Ayotte and Gil Alexandrowicz}, doi = {10.1021/acs.jpcc.6b08746}, year = {2016}, date = {2016-10-13}, journal = {The Journal of Physical Chemistry C}, volume = {120}, number = {44}, pages = {25445-25450}, abstract = {In situ NMR spin–lattice relaxation measurements were performed on several vapor deposited ices. The measurements, which span more than 6 orders of magnitude in relaxation times, show a complex spin–lattice relaxation pattern that is strongly dependent on the growth conditions of the sample. The relaxation patterns change from multitime scale relaxation for samples grown at temperatures below the amorphous–crystalline transition temperature to single exponential recovery for samples grown above the transition temperature. The slow-relaxation contribution seen in cold-grown samples exhibits a temperature dependence, and becomes even slower after the sample is annealed at 200 K. The fast-relaxation contribution seen in these samples, does not seem to change or disappear even when heating to temperatures where the sample is evaporated. The possibility that the fast relaxation component is linked to the microporous structures in amorphous ice samples is further examined using an environmental electron scanning microscope. The images reveal complex mesoscale microporous structures which maintain their morphology up to their desorption temperatures. These findings, support the possibility that water molecules at pore surfaces might be responsible for the fast-relaxation contribution. Furthermore, the results of this study indicate that the pore-collapse dynamics observed in the past in amorphous ices using other experimental techniques, might be effectively inhibited in samples which are grown by relatively fast vapor deposition.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In situ NMR spin–lattice relaxation measurements were performed on several vapor deposited ices. The measurements, which span more than 6 orders of magnitude in relaxation times, show a complex spin–lattice relaxation pattern that is strongly dependent on the growth conditions of the sample. The relaxation patterns change from multitime scale relaxation for samples grown at temperatures below the amorphous–crystalline transition temperature to single exponential recovery for samples grown above the transition temperature. The slow-relaxation contribution seen in cold-grown samples exhibits a temperature dependence, and becomes even slower after the sample is annealed at 200 K. The fast-relaxation contribution seen in these samples, does not seem to change or disappear even when heating to temperatures where the sample is evaporated. The possibility that the fast relaxation component is linked to the microporous structures in amorphous ice samples is further examined using an environmental electron scanning microscope. The images reveal complex mesoscale microporous structures which maintain their morphology up to their desorption temperatures. These findings, support the possibility that water molecules at pore surfaces might be responsible for the fast-relaxation contribution. Furthermore, the results of this study indicate that the pore-collapse dynamics observed in the past in amorphous ices using other experimental techniques, might be effectively inhibited in samples which are grown by relatively fast vapor deposition. |
2015 |
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 | Guillaume Marcotte, Patrick Marchand, Stephanie Pronovost, Patrick Ayotte, Carine Laffon, Philippe Parent Surface-Enhanced Nitrate Photolysis on Ice Article de journal The Journal of Physical Chemistry A, 119 (10), p. 1996-2005, 2015. @article{Marcotte2015, title = {Surface-Enhanced Nitrate Photolysis on Ice}, author = {Guillaume Marcotte and Patrick Marchand and Stephanie Pronovost and Patrick Ayotte and Carine Laffon and Philippe Parent}, doi = {10.1021/jp511173w}, year = {2015}, date = {2015-02-11}, journal = {The Journal of Physical Chemistry A}, volume = {119}, number = {10}, pages = {1996-2005}, abstract = {Heterogeneous nitrate photolysis is the trigger for many chemical processes occurring in the polar boundary layer and is widely believed to occur in a quasi-liquid layer (QLL) at the surface of ice. The dipole-forbidden character of the electronic transition relevant to boundary layer atmospheric chemistry and the small photolysis/photoproduct yields in ice (and in water) may confer a significant enhancement and interfacial specificity to this important photochemical reaction at the surface of ice. Using amorphous solid water films at cryogenic temperatures as models for the disordered interstitial air–ice interface within the snowpack suppresses the diffusive uptake kinetics, thereby prolonging the residence time of nitrate anions at the surface of ice. This approach allows their slow heterogeneous photolysis kinetics to be studied, providing the first direct evidence that nitrates adsorbed onto the first molecular layer at the surface of ice are photolyzed more effectively than those dissolved within the bulk. Vibrational spectroscopy allows the ∼3-fold enhancement in photolysis rates to be correlated with the nitrates’ distorted intramolecular geometry, thereby hinting at the role played by the greater chemical heterogeneity in their solvation environment at the surface of ice than that in the bulk. A simple 1D kinetic model suggests (1) that a 3(6)-fold enhancement in photolysis rate for nitrates adsorbed onto the ice surface could increase the photochemical NO2 emissions from a 5(8) nm thick photochemically active interfacial layer by 30(60)%, and (2) that 25(40)% of the NO2 photochemical emissions to the snowpack interstitial air are released from the topmost molecularly thin surface layer on ice. These findings may provide a new paradigm for heterogeneous (photo)chemistry at temperatures below those required for a QLL to form at the ice surface.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Heterogeneous nitrate photolysis is the trigger for many chemical processes occurring in the polar boundary layer and is widely believed to occur in a quasi-liquid layer (QLL) at the surface of ice. The dipole-forbidden character of the electronic transition relevant to boundary layer atmospheric chemistry and the small photolysis/photoproduct yields in ice (and in water) may confer a significant enhancement and interfacial specificity to this important photochemical reaction at the surface of ice. Using amorphous solid water films at cryogenic temperatures as models for the disordered interstitial air–ice interface within the snowpack suppresses the diffusive uptake kinetics, thereby prolonging the residence time of nitrate anions at the surface of ice. This approach allows their slow heterogeneous photolysis kinetics to be studied, providing the first direct evidence that nitrates adsorbed onto the first molecular layer at the surface of ice are photolyzed more effectively than those dissolved within the bulk. Vibrational spectroscopy allows the ∼3-fold enhancement in photolysis rates to be correlated with the nitrates’ distorted intramolecular geometry, thereby hinting at the role played by the greater chemical heterogeneity in their solvation environment at the surface of ice than that in the bulk. A simple 1D kinetic model suggests (1) that a 3(6)-fold enhancement in photolysis rate for nitrates adsorbed onto the ice surface could increase the photochemical NO2 emissions from a 5(8) nm thick photochemically active interfacial layer by 30(60)%, and (2) that 25(40)% of the NO2 photochemical emissions to the snowpack interstitial air are released from the topmost molecularly thin surface layer on ice. These findings may provide a new paradigm for heterogeneous (photo)chemistry at temperatures below those required for a QLL to form at the ice surface. |
2013 |
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 | Guillaume Marcotte, Patrick Ayotte, Azzedine Bendounan, Fausto Sirotti, Carine Laffon, Philippe Parent Dissociative Adsorption of Nitric Acid at the Surface of Amorphous Solid Water Revealed by X-ray Absorption Spectroscopy Article de journal The Journal of Physical Chemistry Letters, 4 (16), p. 2643-2648, 2013. @article{Marcotte2013, title = {Dissociative Adsorption of Nitric Acid at the Surface of Amorphous Solid Water Revealed by X-ray Absorption Spectroscopy}, author = {Guillaume Marcotte and Patrick Ayotte and Azzedine Bendounan and Fausto Sirotti and Carine Laffon and Philippe Parent }, doi = {10.1021/jz401310j}, year = {2013}, date = {2013-07-24}, journal = {The Journal of Physical Chemistry Letters}, volume = {4}, number = {16}, pages = {2643-2648}, abstract = {A spectral feature unique to the molecularly adsorbed state of HNO3 is found in X-ray absorption spectroscopy. This distinctive signature reveals the extent to which nitric acid is ionically dissociated upon its adsorption on amorphous solid water (ASW) at low coverage and low temperature. Thermal annealing induces irreversible proton transfer from HNO3(ads), demonstrating that it is metastable with respect to ionic dissociation below 100 K at the surface of ASW. The slight decrease in ionic dissociation propensity reported for nitric acid at liquid water surfaces thus appears to be overwhelmed by the strong exothermicity of this reaction as its entropic inhibition becomes increasingly suppressed the lower the temperature. These findings may be relevant for atmospheric chemistry processes involving nitric acid, which should thus, according to thermodynamic considerations, be expected to behave as a strong acid at the surface of supercooled aerosols and of the quasi-liquid layer.}, keywords = {}, pubstate = {published}, tppubtype = {article} } A spectral feature unique to the molecularly adsorbed state of HNO3 is found in X-ray absorption spectroscopy. This distinctive signature reveals the extent to which nitric acid is ionically dissociated upon its adsorption on amorphous solid water (ASW) at low coverage and low temperature. Thermal annealing induces irreversible proton transfer from HNO3(ads), demonstrating that it is metastable with respect to ionic dissociation below 100 K at the surface of ASW. The slight decrease in ionic dissociation propensity reported for nitric acid at liquid water surfaces thus appears to be overwhelmed by the strong exothermicity of this reaction as its entropic inhibition becomes increasingly suppressed the lower the temperature. These findings may be relevant for atmospheric chemistry processes involving nitric acid, which should thus, according to thermodynamic considerations, be expected to behave as a strong acid at the surface of supercooled aerosols and of the quasi-liquid layer. |
2012 |
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 | Pierre-Alexandre Turgeon, Patrick Ayotte, Elina Lisitsin, Yossi Meir, Tatyana Kravchuk, Gil Alexandrowicz Preparation, isolation, storage, and spectroscopic characterization of water vapor enriched in the ortho-H2O nuclear spin isomer Article de journal Physical Review A, 86 (6), p. 062710, 2012. @article{Turgeon2012, title = {Preparation, isolation, storage, and spectroscopic characterization of water vapor enriched in the ortho-H2O nuclear spin isomer}, author = {Pierre-Alexandre Turgeon and Patrick Ayotte and Elina Lisitsin and Yossi Meir and Tatyana Kravchuk and Gil Alexandrowicz}, doi = {10.1103/PhysRevA.86.062710}, year = {2012}, date = {2012-12-12}, journal = {Physical Review A}, volume = {86}, number = {6}, pages = {062710}, abstract = {Using magnetic focusing in a supersonic jet, a beam of "normal" H2O molecules seeded in a krypton carrier gas is shown to provide a source of water molecules that is highly enhanced in the ortho-H2O (o-H2O) nuclear spin isomer over the high-temperature equilibrium 3:1 ortho:para ratio. Water from the magnetically focused beam is then isolated and stored within a Kr matrix at 13 K whereby the amplitude and lifetime of this strong nuclear spin polarization are quantified spectroscopically. Attempts to store the polarization in a colder Kr matrix reveal complex nuclear spin conversion processes.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Using magnetic focusing in a supersonic jet, a beam of "normal" H2O molecules seeded in a krypton carrier gas is shown to provide a source of water molecules that is highly enhanced in the ortho-H2O (o-H2O) nuclear spin isomer over the high-temperature equilibrium 3:1 ortho:para ratio. Water from the magnetically focused beam is then isolated and stored within a Kr matrix at 13 K whereby the amplitude and lifetime of this strong nuclear spin polarization are quantified spectroscopically. Attempts to store the polarization in a colder Kr matrix reveal complex nuclear spin conversion processes. |
 | Patrick Marchand, Guillaume Marcotte, Patrick Ayotte Spectroscopic Study of HNO3 Dissociation on Ice Article de journal The Journal of Physical Chemistry A, 1169 (49), p. 12112-121222, 2012. @article{Marchand2012, title = {Spectroscopic Study of HNO3 Dissociation on Ice}, author = {Patrick Marchand and Guillaume Marcotte and Patrick Ayotte}, doi = {10.1021/jp309533f}, year = {2012}, date = {2012-11-06}, journal = {The Journal of Physical Chemistry A}, volume = {1169}, number = {49}, pages = {12112-121222}, abstract = {A detailed spectroscopic study of HNO3:H2O binary amorphous mixtures, and of the adsorption of HNO3 onto ice, is reported. Using a classical optics model, the extent of intermixing and of ionic dissociation of adsorbed HNO3, which forms a strong acid with liquid water, is determined as a function of HNO3 coverage and temperature. Even at temperatures as low as 45 K, where intermixing is limited to at most a few molecular layers at the interface, ionic dissociation of adsorbed HNO3 is observed to be extensive. While some amount of molecularly adsorbed HNO3 is observed at the surface of ice at 45 K, its ionic dissociation occurs irreversibly upon heating the ice substrate to 120 K. The molecularly adsorbed state of HNO3 is not restored upon cooling, suggesting HNO3 is a metastable entity at the surface of ice. Therefore, despite ionic dissociation of HNO3 being thermodynamically favored, it appears to be kinetically inhibited at the surface of amorphous solid water at temperatures below 120 K.}, keywords = {}, pubstate = {published}, tppubtype = {article} } A detailed spectroscopic study of HNO3:H2O binary amorphous mixtures, and of the adsorption of HNO3 onto ice, is reported. Using a classical optics model, the extent of intermixing and of ionic dissociation of adsorbed HNO3, which forms a strong acid with liquid water, is determined as a function of HNO3 coverage and temperature. Even at temperatures as low as 45 K, where intermixing is limited to at most a few molecular layers at the interface, ionic dissociation of adsorbed HNO3 is observed to be extensive. While some amount of molecularly adsorbed HNO3 is observed at the surface of ice at 45 K, its ionic dissociation occurs irreversibly upon heating the ice substrate to 120 K. The molecularly adsorbed state of HNO3 is not restored upon cooling, suggesting HNO3 is a metastable entity at the surface of ice. Therefore, despite ionic dissociation of HNO3 being thermodynamically favored, it appears to be kinetically inhibited at the surface of amorphous solid water at temperatures below 120 K. |
2011 |
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 | Patrick Ayotte, Patrick Marchand, John L. Daschbach, R. Scott Smith, Bruce D. Kay HCl Adsorption and Ionization on Amorphous and Crystalline H2O Films below 50 K Article de journal The Journal of Physical Chemistry A, 115 (23), p. 6002-6014, 2011. @article{Ayotte2011, title = {HCl Adsorption and Ionization on Amorphous and Crystalline H2O Films below 50 K}, author = {Patrick Ayotte and Patrick Marchand and John L. Daschbach and R. Scott Smith and Bruce D. Kay}, doi = {10.1021/jp110398j}, year = {2011}, date = {2011-04-20}, journal = {The Journal of Physical Chemistry A}, volume = {115}, number = {23}, pages = {6002-6014}, abstract = {Molecular beams were used to grow amorphous and crystalline H2O films and to dose HCl upon their surface. The adsorption state of HCl on the ice films was probed with infrared spectroscopy. A Zundel continuum is clearly observed for exposures up to the saturation HCl coverage on ice upon which features centered near 2530, 2120, 1760, and 1220 cm−1 are superimposed. The band centered near 2530 cm−1 is observed only when the HCl adlayer is in direct contact with amorphous solid water or crystalline ice films at temperatures as low as 20 K. The spectral signature of solid HCl (amorphous or crystalline) was identified only after saturation of the adsorption sites in the first layer or when HCl was deposited onto a rare gas spacer layer between the HCl and ice film. These observations strongly support conclusions from recent electron spectroscopy work that reported ionic dissociation of the first layer HCl adsorbed onto the ice surface is spontaneous.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Molecular beams were used to grow amorphous and crystalline H2O films and to dose HCl upon their surface. The adsorption state of HCl on the ice films was probed with infrared spectroscopy. A Zundel continuum is clearly observed for exposures up to the saturation HCl coverage on ice upon which features centered near 2530, 2120, 1760, and 1220 cm−1 are superimposed. The band centered near 2530 cm−1 is observed only when the HCl adlayer is in direct contact with amorphous solid water or crystalline ice films at temperatures as low as 20 K. The spectral signature of solid HCl (amorphous or crystalline) was identified only after saturation of the adsorption sites in the first layer or when HCl was deposited onto a rare gas spacer layer between the HCl and ice film. These observations strongly support conclusions from recent electron spectroscopy work that reported ionic dissociation of the first layer HCl adsorbed onto the ice surface is spontaneous. |
 | Guillaume Marcotte, Patrick Ayotte The Journal of Chemical Physics, 134 (11), p. 114522, 2011. @article{Marcotte2011, title = {Strong intermolecular coupling between the HF stretching and H2O bending vibrations in HF:H2O binary amorphous solids: Breakdown of the electrostatic description of the hydrogen bond}, author = {Guillaume Marcotte and Patrick Ayotte}, doi = {10.1063/1.3562368}, year = {2011}, date = {2011-03-18}, journal = {The Journal of Chemical Physics}, volume = {134}, number = {11}, pages = {114522}, abstract = {The coupling mechanism between the HF stretching and H2O bending vibrations observed in the infrared spectra of HF:H2O binary amorphous solids is analyzed using a simple cluster model. The intermolecular vibrational coupling derived from electrostatic potentials is one order of magnitude smaller, and of the opposite sign, than that obtained from electronic structure-based potentials. This highlights the distinctively covalent character of strong H-bonds and unveils fundamental weaknesses of electrostatic descriptions of vibrational energy transfer in liquid water and aqueous solutions.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The coupling mechanism between the HF stretching and H2O bending vibrations observed in the infrared spectra of HF:H2O binary amorphous solids is analyzed using a simple cluster model. The intermolecular vibrational coupling derived from electrostatic potentials is one order of magnitude smaller, and of the opposite sign, than that obtained from electronic structure-based potentials. This highlights the distinctively covalent character of strong H-bonds and unveils fundamental weaknesses of electrostatic descriptions of vibrational energy transfer in liquid water and aqueous solutions. |
2009 |
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 | Patrick Ayotte, Zohreh Rafiei, François Porzio, Patrick Marchand Dissociative adsorption of hydrogen fluoride onto amorphous solid water Article de journal The Journal of Chemical Physics, 131 (12), p. 124517, 2009. @article{Ayotte2009, title = {Dissociative adsorption of hydrogen fluoride onto amorphous solid water}, author = {Patrick Ayotte and Zohreh Rafiei and François Porzio and Patrick Marchand}, doi = {10.1063/1.3231999}, year = {2009}, date = {2009-09-30}, journal = {The Journal of Chemical Physics}, volume = {131}, number = {12}, pages = {124517}, abstract = {Adsorption of hydrogen fluoride (HF) onto amorphous solid water films at 50 K is reported to yield a strong absorbance continuum in their reflection-absorption infrared spectra (RAIRS). This and other complex features observed in the RAIRS spectra of stratified binary composite HF:H2O nanoscopic films deposited onto Pt(111) are interpreted quantitatively using a classical optics model. Comparison with experimental data allows us to determine that the absorbance continuum is due to absorption within the film (as opposed to trivial optical effects) and that the extent of intermixing and uptake is mostly limited to the first few molecular layers. Furthermore, extensive isotope scrambling is demonstrated by the observation of similar Zundel continua upon codeposition of neat HF, or DF, and H2O vapors onto Pt(111) at 50 K. These observations are consistent with those expected from extensive ionic dissociation of HF upon dissolution within, and adsorption onto, ASW at 50 K.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Adsorption of hydrogen fluoride (HF) onto amorphous solid water films at 50 K is reported to yield a strong absorbance continuum in their reflection-absorption infrared spectra (RAIRS). This and other complex features observed in the RAIRS spectra of stratified binary composite HF:H2O nanoscopic films deposited onto Pt(111) are interpreted quantitatively using a classical optics model. Comparison with experimental data allows us to determine that the absorbance continuum is due to absorption within the film (as opposed to trivial optical effects) and that the extent of intermixing and uptake is mostly limited to the first few molecular layers. Furthermore, extensive isotope scrambling is demonstrated by the observation of similar Zundel continua upon codeposition of neat HF, or DF, and H2O vapors onto Pt(111) at 50 K. These observations are consistent with those expected from extensive ionic dissociation of HF upon dissolution within, and adsorption onto, ASW at 50 K. |
 | François Cholette, Tykhon Zubkov, R. Scott Smith, Zdenek Dohnálek, Bruce D. Kay, Patrick Ayotte Infrared Spectroscopy and Optical Constants of Porous Amorphous Solid Water Article de journal The Journal of Physical Chemistry B, 113 (13), p. 4131-4140, 2009. @article{Cholette2009, title = {Infrared Spectroscopy and Optical Constants of Porous Amorphous Solid Water}, author = {François Cholette and Tykhon Zubkov and R. Scott Smith and Zdenek Dohnálek and Bruce D. Kay and Patrick Ayotte }, doi = {10.1021/jp806738a}, year = {2009}, date = {2009-04-02}, journal = {The Journal of Physical Chemistry B}, volume = {113}, number = {13}, pages = {4131-4140}, abstract = {Reflection−absorption infrared spectra (RAIRS) of amorphous solid water (ASW) films grown at 20 K on a Pt(111) substrate at various angles (θBeam = 0−85°) using a molecular beam are reported. They display complex features arising from the interplay between refraction, absorption within the sample, and interference effects between the multiple reflections at the film−substrate and film−vacuum interfaces. Using a simple classical optics model based on Fresnel equations, we obtain optical constants [i.e., n(ω) and k(ω)] for porous ASW in the 1000−4000 cm−1 (10−2.5 μm) range. The behavior of the optical properties of ASW in the intramolecular OH stretching region with increasing θBeam is shown to be strongly correlated with its decreasing density and increasing surface area. A direct comparison between the RAIRS and calculated vibrational spectra shows a large difference (∼200 cm−1) in the position of the coupled H-bonded intramolecular OH stretching vibrations spectral feature. Moreover, this band shifts in opposite directions with increasing θBeam in RAIRS and vibrational spectra demonstrating RAIRS spectra cannot be interpreted straightforwardly as vibrational spectra due to severe optical distortions from refraction and interference effects.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Reflection−absorption infrared spectra (RAIRS) of amorphous solid water (ASW) films grown at 20 K on a Pt(111) substrate at various angles (θBeam = 0−85°) using a molecular beam are reported. They display complex features arising from the interplay between refraction, absorption within the sample, and interference effects between the multiple reflections at the film−substrate and film−vacuum interfaces. Using a simple classical optics model based on Fresnel equations, we obtain optical constants [i.e., n(ω) and k(ω)] for porous ASW in the 1000−4000 cm−1 (10−2.5 μm) range. The behavior of the optical properties of ASW in the intramolecular OH stretching region with increasing θBeam is shown to be strongly correlated with its decreasing density and increasing surface area. A direct comparison between the RAIRS and calculated vibrational spectra shows a large difference (∼200 cm−1) in the position of the coupled H-bonded intramolecular OH stretching vibrations spectral feature. Moreover, this band shifts in opposite directions with increasing θBeam in RAIRS and vibrational spectra demonstrating RAIRS spectra cannot be interpreted straightforwardly as vibrational spectra due to severe optical distortions from refraction and interference effects. |
2008 |
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 | Patrick Ayotte, Sylvain Plessis, Patrick Marchand Trapping proton transfer intermediates in the disordered hydrogen-bonded network of cryogenic hydrofluoric acid solutions Article de journal Physical Chemistry Chemical Physics, 10 , p. 4785-4792, 2008. @article{Ayotte2008, title = {Trapping proton transfer intermediates in the disordered hydrogen-bonded network of cryogenic hydrofluoric acid solutions}, author = {Patrick Ayotte and Sylvain Plessis and Patrick Marchand}, doi = { 10.1039/B806654J}, year = {2008}, date = {2008-06-30}, journal = {Physical Chemistry Chemical Physics}, volume = {10}, pages = {4785-4792}, abstract = {A molecular-level description of the structural and dynamical aspects that are responsible for the weak acid behaviour of dilute hydrofluoric acid solutions and their unusual increased acidity at near equimolar concentrations continues to elude us. We address this problem by reporting reflection–absorption infrared spectra (RAIRS) of cryogenic HF–H2O binary mixtures at various compositions prepared as nanoscopic films using molecular beam techniques. Optical constants for these cryogenic solutions [n(ω) and k(ω)] are obtained by iteratively solving Fresnel equations for stratified media. Modeling of the experimental RAIRS spectra allow for a quantitative interpretation of the complex interplay between multiple reflections, optical interference and absorption effects. The evolution of the strong absorption features in the intermediate 1000–3000 cm−1 range with increasing HF concentration reveals the presence of various ionic dissociation intermediates that are trapped in the disordered H-bonded network of cryogenic hydrofluoric acid solutions. Our findings are discussed in light of the conventional interpretation of why hydrofluoric acid is a weak acid revealing molecular-level details of the mechanism for HF ionization that may be relevant to analogous elementary processes involved in the ionization of weak acids in aqueous solutions.}, keywords = {}, pubstate = {published}, tppubtype = {article} } A molecular-level description of the structural and dynamical aspects that are responsible for the weak acid behaviour of dilute hydrofluoric acid solutions and their unusual increased acidity at near equimolar concentrations continues to elude us. We address this problem by reporting reflection–absorption infrared spectra (RAIRS) of cryogenic HF–H2O binary mixtures at various compositions prepared as nanoscopic films using molecular beam techniques. Optical constants for these cryogenic solutions [n(ω) and k(ω)] are obtained by iteratively solving Fresnel equations for stratified media. Modeling of the experimental RAIRS spectra allow for a quantitative interpretation of the complex interplay between multiple reflections, optical interference and absorption effects. The evolution of the strong absorption features in the intermediate 1000–3000 cm−1 range with increasing HF concentration reveals the presence of various ionic dissociation intermediates that are trapped in the disordered H-bonded network of cryogenic hydrofluoric acid solutions. Our findings are discussed in light of the conventional interpretation of why hydrofluoric acid is a weak acid revealing molecular-level details of the mechanism for HF ionization that may be relevant to analogous elementary processes involved in the ionization of weak acids in aqueous solutions. |
 | Radu Iftimie, Vibin Thomas, Sylvain Plessis, Patrick Marchand, Patrick Ayotte Spectral Signatures and Molecular Origin of Acid Dissociation Intermediates Article de journal Journal of the American Chemical Society, 130 (18), p. 5901-5907, 2008. @article{Iftimie2008, title = {Spectral Signatures and Molecular Origin of Acid Dissociation Intermediates}, author = {Radu Iftimie and Vibin Thomas and Sylvain Plessis and Patrick Marchand and Patrick Ayotte}, doi = {10.1021/ja077846o}, year = {2008}, date = {2008-04-03}, journal = {Journal of the American Chemical Society}, volume = {130}, number = {18}, pages = {5901-5907}, abstract = {The existence of a broad, mid-infrared absorption ranging from 1000 to 3000 cm−1 is usually interpreted as a signature for the existence of protonated water networks. Herein, we use cryogenic mixtures of water and hydrogen fluoride (HF) and show experimental and computational evidence that similarly wide absorptions can be generated by a broad distribution of proton-shared and ion pair complexes. In the present case, we demonstrate that the broadening is mainly inhomogeneous, reflecting the fact that the topology of the first solvation shell determines the local degree of ionization and the shared-proton asymmetric stretching frequency within H2O·HF complexes. The extreme sensitivity of the proton transfer potential energy hypersurface to local hydrogen bonding topologies modulates its vibrational frequency from 2800 down to ∼1300 cm−1, the latter value being characteristic of solvation geometries that yield similar condensed-phase proton affinities for H2O and fluoride. By linking the local degree of ionization to the solvation pattern, we are able to propose a mechanism of ionization for HF in aqueous solutions and to explain some of their unusual properties at large concentrations. However, an important conclusion of broad scientific interest is our prediction that spectral signatures that are normally attributed to protonated water networks could also reveal the presence of strong hydrogen bonds between un-ionized acids and water molecules, with important consequences to spectroscopic investigations of biologically relevant proton channels and pumps}, keywords = {}, pubstate = {published}, tppubtype = {article} } The existence of a broad, mid-infrared absorption ranging from 1000 to 3000 cm−1 is usually interpreted as a signature for the existence of protonated water networks. Herein, we use cryogenic mixtures of water and hydrogen fluoride (HF) and show experimental and computational evidence that similarly wide absorptions can be generated by a broad distribution of proton-shared and ion pair complexes. In the present case, we demonstrate that the broadening is mainly inhomogeneous, reflecting the fact that the topology of the first solvation shell determines the local degree of ionization and the shared-proton asymmetric stretching frequency within H2O·HF complexes. The extreme sensitivity of the proton transfer potential energy hypersurface to local hydrogen bonding topologies modulates its vibrational frequency from 2800 down to ∼1300 cm−1, the latter value being characteristic of solvation geometries that yield similar condensed-phase proton affinities for H2O and fluoride. By linking the local degree of ionization to the solvation pattern, we are able to propose a mechanism of ionization for HF in aqueous solutions and to explain some of their unusual properties at large concentrations. However, an important conclusion of broad scientific interest is our prediction that spectral signatures that are normally attributed to protonated water networks could also reveal the presence of strong hydrogen bonds between un-ionized acids and water molecules, with important consequences to spectroscopic investigations of biologically relevant proton channels and pumps |
2006 |
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 | Patrick Marchand, Samuel Riou, Patrick Ayotte Diffusion of methanol in polycrystalline ice Article de journal The Journal of Physical Chemistry A, 110 (11), p. 654-664, 2006. @article{Marchand2006, title = { Diffusion of methanol in polycrystalline ice}, author = {Patrick Marchand and Samuel Riou and Patrick Ayotte}, doi = {10.1021/jp0640878}, year = {2006}, date = {2006-09-26}, journal = {The Journal of Physical Chemistry A}, volume = {110}, number = {11}, pages = {654-664}, abstract = {Quantitative analyses of the isothermal desorption kinetics from methanol-doped H2O films on Pt(111) reveal that transport kinetics for CH3OH in polycrystalline ice are much slower than previously reported. They also indicate that MeOH displays first-order desorption kinetics with respect to its instantaneous surface concentration below 0.1 mole fraction in ice. These observations allow isothermal desorption rate measurements to be interpreted in terms of a depth profiling analysis providing one-dimensional concentration depth profiles from methanol-doped polycrystalline ice films. Using a straightforward approach to inhibit ice sublimation, transport properties are extracted from the evolution of concentration depth profiles obtained after thermal annealing of binary ice films at high temperature. Heterodiffusion coefficients for methanol in polycrystalline (cubic) ice Ic films are reported for temperatures between 145 and 195 K and for concentrations below 10-3 mole fraction. Finally, diffusion kinetics for methanol in ice are shown to display a very strong concentration dependence that may contribute, in addition to variations in laboratory samples microstructure, to the disagreements reported in the literature regarding the transport properties of ice. }, keywords = {}, pubstate = {published}, tppubtype = {article} } Quantitative analyses of the isothermal desorption kinetics from methanol-doped H2O films on Pt(111) reveal that transport kinetics for CH3OH in polycrystalline ice are much slower than previously reported. They also indicate that MeOH displays first-order desorption kinetics with respect to its instantaneous surface concentration below 0.1 mole fraction in ice. These observations allow isothermal desorption rate measurements to be interpreted in terms of a depth profiling analysis providing one-dimensional concentration depth profiles from methanol-doped polycrystalline ice films. Using a straightforward approach to inhibit ice sublimation, transport properties are extracted from the evolution of concentration depth profiles obtained after thermal annealing of binary ice films at high temperature. Heterodiffusion coefficients for methanol in polycrystalline (cubic) ice Ic films are reported for temperatures between 145 and 195 K and for concentrations below 10-3 mole fraction. Finally, diffusion kinetics for methanol in ice are shown to display a very strong concentration dependence that may contribute, in addition to variations in laboratory samples microstructure, to the disagreements reported in the literature regarding the transport properties of ice. |
2005 |
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 | Patrick Ayotte, Martin Hébert, Patrick Marchand Why is hydrofluoric acid a weak acid? Article de journal The Journal of Chemical Physics, 123 (18), p. 184501, 2005. @article{Ayotte2005, title = {Why is hydrofluoric acid a weak acid?}, author = {Patrick Ayotte and Martin Hébert and Patrick Marchand}, doi = {10.1063/1.2090259}, year = {2005}, date = {2005-11-04}, journal = {The Journal of Chemical Physics}, volume = {123}, number = {18}, pages = {184501}, abstract = {The infrared vibrational spectra of amorphous solid water thin films doped with HF at 40K reveal a strong continuous absorbance in the 1000–3275cm−1 range. This so-called Zundel continuum is the spectroscopic hallmark for aqueous protons. The extensive ionic dissociation of HF at such low temperature suggests that the reaction enthalpy remains negative down to 40K. These observations support the interpretation that dilute HF aqueous solutions behave as weak acids largely due to the large positive reaction entropy resulting from the structure making character of the hydrated fluoride ion.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The infrared vibrational spectra of amorphous solid water thin films doped with HF at 40K reveal a strong continuous absorbance in the 1000–3275cm−1 range. This so-called Zundel continuum is the spectroscopic hallmark for aqueous protons. The extensive ionic dissociation of HF at such low temperature suggests that the reaction enthalpy remains negative down to 40K. These observations support the interpretation that dilute HF aqueous solutions behave as weak acids largely due to the large positive reaction entropy resulting from the structure making character of the hydrated fluoride ion. |
