Swapan Roychowdhury | NISER - Academia.edu (original) (raw)

Papers by Swapan Roychowdhury

Physical Review Letters, Feb 19, 2003

Vibrational dephasing of the nitrogen molecule is known to show highly interesting anomalies near... more Vibrational dephasing of the nitrogen molecule is known to show highly interesting anomalies near its gas-liquid critical point. Here we present theoretical and computational studies of the Raman linewidth of nitrogen along the critical isochore. The linewidth is found to have a lambda-shaped temperature dependence near the critical point. As observed in experimental studies, the calculated line shape becomes Gaussian as the critical temperature (T c) is approached. Both the present simulation and a mode coupling theory analysis show that the slow decay of the enhanced density fluctuations near the critical point, probed at the subpicosecond time scales by vibrational frequency modulation, along with an enhanced vibration-rotation coupling, are the main causes of the observed anomalies.

Journal of Chemical Physics, Jul 24, 2003

We present results of extensive computer simulations and theoretical analysis of vibrational phas... more We present results of extensive computer simulations and theoretical analysis of vibrational phase relaxation of a nitrogen molecule along the critical isochore and also along the gas-liquid coexistence. The simulation includes all the different contributions [atom-atom (AA), vibrationrotation (VR) and resonant transfer] and their cross-correlations. Following Everitt and Skinner, we have included the vibrational coordinate (q) dependence of the interatomic potential. It is found that the latter makes an important contribution. The simulated results are in good agreement with the experiments. Dephasing time (τ v) and the root mean square frequency fluctuation (∆) in the supercritical region are calculated. The principal important results are: (a) a crossover from a Lorentzian-type to a Gaussian line shape is observed as the critical point is approached along the isochore (from above), (b) the root mean square frequency fluctuation shows nonmonotonic dependence on the temperature along critical isochore, (c) along the coexistence line and the critical isochore the temperature dependent linewidth shows a divergence-like λ-shape behavior, and (d) the value of the critical exponents along the coexistence and along the isochore are obtained by fitting. It is found that the linewidths (directly proportional to the rate of vibrational phase relaxation) calculated from the time integral of the normal coordinate time correlation function [C Q (t)] are in good agreement with the known experimental results. The origin of the anomalous temperature dependence of linewidth can be traced to simultaneous occurrence of several factors, (i) the enhancement of negative cross-correlations between AA and VR contributions and (ii) the large density fluctuations as the critical point (CP) is approached. The former makes the decay faster so that local density fluctuations are probed on a femtosecond time scale. The reason for the negative cross-correlation between AA and VR is explored in detail. A mode coupling theory (MCT) analysis shows the slow decay of the enhanced density fluctuations near critical point. The MCT analysis demonstrates that the large enhancement of VR coupling near CP arises from the non-Gaussian behavior of density fluctuation and this enters through a nonzero value of the triplet direct correlation function.

arXiv (Cornell University), Oct 8, 2002

Vibrational dephasing of nitrogen molecule is known to show highly interesting anomalies near its... more Vibrational dephasing of nitrogen molecule is known to show highly interesting anomalies near its gas-liquid critical point. Here we present theoretical and computational studies of the Raman linewidth of nitrogen along the critical isochore. The linewidth is found to have a lambda shaped temperature dependence near the critical point. As observed in experimental studies, the calculated lineshape becomes Gaussian as the critical temperature (Tc) is approached. Both the present simulation and a mode coupling theory (MCT) analysis show that the slow decay of the enhanced density fluctuations near the critical point (CP), probed at the sub-picosecond time scales by vibrational frequency modulation, along with an enhanced vibration-rotation coupling, are the main causes of the observed anomalies.

Journal of Chemical Physics, Apr 8, 2005

We present here isothermal-isobaric N-P-T ensemble molecular dynamics simulations of vibrational ... more We present here isothermal-isobaric N-P-T ensemble molecular dynamics simulations of vibrational phase relaxation in a model system to explore the unusual features arising due to concentration fluctuations which are absent in one component systems. The model studied consider strong attractive interaction between the dissimilar species to discourage phase separation. The model reproduces the experimentally observed nonmonotonic, nearly symmetric, composition dependence of the dephasing rate. In addition, several other experimentally observed features, such as the maximum of the frequency modulation correlation time c at mole fraction near 0.5 and the maximum rate enhancement by a factor of about 3 above the pure component value, are also reproduced. The product of mean square frequency modulation ͓͗⌬ 2 ͑0͔͒͘ with c indicates that the present model is in the intermediate regime of inhomogeneous broadening. The nonmonotonic composition A dependence of the dephasing time v is found to be primarily due to the nonmonotonic dependence of c , rather than due to a similar dependence in the amplitude of ͗⌬ 2 ͑0͒͘. The probability distribution of ⌬ shows a markedly non-Gaussian behavior at intermediate composition ͑ A Ӎ 0.5͒. We have also calculated the composition dependence of the viscosity in order to explore the correlation between the composition dependence of viscosity * with that of v and c. It is found that both the correlation time essentially follow the composition dependence of the viscosity. A mode coupling theory is presented to include the effects of composition fluctuations in binary mixture.

The Journal of Chemical Physics, 2005

The Journal of Chemical Physics, 2003

Chemical Physics, 2008

The ultrafast vibrational phase relaxation of O-H stretch in bulk water is investigated in molecu... more The ultrafast vibrational phase relaxation of O-H stretch in bulk water is investigated in molecular dynamics simulations. The dephasing time (T 2) of the O-H stretch in bulk water calculated from the frequency fluctuation time correlation function (C x (t)) is in the range of 70-80 femtosecond (fs), which is comparable to the characteristic timescale obtained from the vibrational echo peak shift measurements using infrared photon echo [W.P. de Boeij, M.S. Pshenichnikov, D.A. Wiersma, Ann. Rev. Phys. Chem. 49 (1998) 99]. The ultrafast decay of C x (t) is found to be responsible for the ultrashort T 2 in bulk water. Careful analysis reveals the following two interesting reasons for the ultrafast decay of C x (t). (A) The large amplitude angular jumps of water molecules (within 30-40 fs time duration) provide a large scale contribution to the mean square vibrational frequency fluctuation and gives rise to the rapid spectral diffusion on 100 fs time scale. (B) The projected force, due to all the atoms of the solvent molecules on the oxygen (F O (t)) and hydrogen (F H (t)) atom of the O-H bond exhibit a large negative cross-correlation (NCC). We further find that this NCC is partly responsible for a weak, non-Arrhenius temperature dependence of the dephasing rate.

Chemical Physics, 2008

Vibrational dephasing of the nitrogen molecule is known to show highly interesting anomalies near... more Vibrational dephasing of the nitrogen molecule is known to show highly interesting anomalies near its gas-liquid critical point. Here we present theoretical and computational studies of the Raman linewidth of nitrogen along the critical isochore. The linewidth is found to have a lambda-shaped temperature dependence near the critical point. As observed in experimental studies, the calculated line shape becomes Gaussian as the critical temperature (Tc) is approached. Both the present simulation and a mode coupling theory analysis show that the slow decay of the enhanced density fluctuations near the critical point, probed at the subpicosecond time scales by vibrational frequency modulation, along with an enhanced vibration-rotation coupling, are the main causes of the observed anomalies.

The ultrafast vibrational phase relaxation of O–H stretch in bulk water is investigated in molecu... more The ultrafast vibrational phase relaxation of O–H stretch in bulk water is investigated in molecular dynamics simulations. The dephasing time (T 2) of the O–H stretch in bulk water calculated from the frequency fluctuation time correlation function (C x (t)) is in the range of 70–80 femtosecond (fs), which is comparable to the characteristic timescale obtained from the vibrational echo peak shift measurements using infrared photon echo [W.P. de Boeij, M.S. Pshenichnikov, D.A. Wiersma, Ann. Rev. Phys. Chem. 49 (1998) 99]. The ultrafast decay of C x (t) is found to be responsible for the ultrashort T 2 in bulk water. Careful analysis reveals the following two interesting reasons for the ultrafast decay of C x (t). (A) The large amplitude angular jumps of water molecules (within 30–40 fs time duration) provide a large scale contribution to the mean square vibrational frequency fluctuation and gives rise to the rapid spectral diffusion on 100 fs time scale. (B) The projected force, due to all the atoms of the solvent molecules on the oxygen (F O (t)) and hydrogen (F H (t)) atom of the O–H bond exhibit a large negative cross-correlation (NCC). We further find that this NCC is partly responsible for a weak, non-Arrhenius temperature dependence of the dephasing rate.

We present here isothermal-isobaric N-P-T ensemble molecular dynamics simulations of vibrational ... more We present here isothermal-isobaric N-P-T ensemble molecular dynamics simulations of vibrational phase relaxation in a model system to explore the unusual features arising due to concentration fluctuations which are absent in one component systems. The model studied consider strong attractive interaction between the dissimilar species to discourage phase separation. The model reproduces the experimentally observed nonmonotonic, nearly symmetric, composition dependence of the dephasing rate. In addition, several other experimentally observed features, such as the maximum of the frequency modulation correlation time c at mole fraction near 0.5 and the maximum rate enhancement by a factor of about 3 above the pure component value, are also reproduced. The product of mean square frequency modulation 2 0 with c indicates that the present model is in the intermediate regime of inhomogeneous broadening. The nonmonotonic composition A dependence of the dephasing time v is found to be primarily due to the nonmonotonic dependence of c , rather than due to a similar dependence in the amplitude of 2 0. The probability distribution of shows a markedly non-Gaussian behavior at intermediate composition A 0.5. We have also calculated the composition dependence of the viscosity in order to explore the correlation between the composition dependence of viscosity * with that of v and c. It is found that both the correlation time essentially follow the composition dependence of the viscosity. A mode coupling theory is presented to include the effects of composition fluctuations in binary mixture.

We present results of extensive computer simulations and theoretical analysis of vibrational phas... more We present results of extensive computer simulations and theoretical analysis of vibrational phase relaxation of a nitrogen molecule along the critical isochore and also along the gas-liquid coexistence. The simulation includes all the different contributions atom-atom AA, vibration-rotation VR, and resonant transfer and their cross-correlations. Following Everitt and Skinner, we have included the vibrational coordinate (q) dependence of the interatomic potential. It is found that the latter makes an important contribution. The simulated results are in good agreement with the experiments. Dephasing time (v) and the root mean square frequency fluctuation in the supercritical region are calculated. The principal important results are: a a crossover from a Lorentzian-type to a Gaussian lineshape is observed as the critical point is approached along the isochore from above, b the root mean square frequency fluctuation shows nonmonotonic dependence on the temperature along critical isochore, c along the coexistence line and the critical isochore the temperature dependent linewidth shows a divergence-like-shape behavior, and d the value of the critical exponents along the coexistence and along the isochore are obtained by fitting. It is found that the linewidths directly proportional to the rate of vibrational phase relaxation calculated from the time integral of the normal coordinate time correlation function C Q (t) are in good agreement with the known experimental results. The origin of the anomalous temperature dependence of linewidth can be traced to simultaneous occurrence of several factors, i the enhancement of negative cross-correlations between AA and VR contributions and ii the large density fluctuations as the critical point CP is approached. The former makes the decay faster so that local density fluctuations are probed on a femtosecond time scale. The reason for the negative cross-correlation between AA and VR is explored in detail. A mode coupling theory MCT analysis shows the slow decay of the enhanced density fluctuations near critical point. The MCT analysis demonstrates that the large enhancement of VR coupling near CP arises from the non-Gaussian behavior of density fluctuation and this enters through a nonzero value of the triplet direct correlation function.

Physical Review Letters, Feb 19, 2003

Journal of Chemical Physics, Jul 24, 2003

arXiv (Cornell University), Oct 8, 2002

Journal of Chemical Physics, Apr 8, 2005

The Journal of Chemical Physics, 2005

The Journal of Chemical Physics, 2003

Chemical Physics, 2008

Vibrational dephasing of the nitrogen molecule is known to show highly interesting anomalies near... more Vibrational dephasing of the nitrogen molecule is known to show highly interesting anomalies near its gas-liquid critical point. Here we present theoretical and computational studies of the Raman linewidth of nitrogen along the critical isochore. The linewidth is found to have a lambda-shaped temperature dependence near the critical point. As observed in experimental studies, the calculated line shape becomes Gaussian as the critical temperature (Tc) is approached. Both the present simulation and a mode coupling theory analysis show that the slow decay of the enhanced density fluctuations near the critical point, probed at the subpicosecond time scales by vibrational frequency modulation, along with an enhanced vibration-rotation coupling, are the main causes of the observed anomalies.

We present here isothermal-isobaric N-P-T ensemble molecular dynamics simulations of vibrational ... more We present here isothermal-isobaric N-P-T ensemble molecular dynamics simulations of vibrational phase relaxation in a model system to explore the unusual features arising due to concentration fluctuations which are absent in one component systems. The model studied consider strong attractive interaction between the dissimilar species to discourage phase separation. The model reproduces the experimentally observed nonmonotonic, nearly symmetric, composition dependence of the dephasing rate. In addition, several other experimentally observed features, such as the maximum of the frequency modulation correlation time c at mole fraction near 0.5 and the maximum rate enhancement by a factor of about 3 above the pure component value, are also reproduced. The product of mean square frequency modulation 2 0 with c indicates that the present model is in the intermediate regime of inhomogeneous broadening. The nonmonotonic composition A dependence of the dephasing time v is found to be primarily due to the nonmonotonic dependence of c , rather than due to a similar dependence in the amplitude of 2 0. The probability distribution of shows a markedly non-Gaussian behavior at intermediate composition A 0.5. We have also calculated the composition dependence of the viscosity in order to explore the correlation between the composition dependence of viscosity * with that of v and c. It is found that both the correlation time essentially follow the composition dependence of the viscosity. A mode coupling theory is presented to include the effects of composition fluctuations in binary mixture.

We present results of extensive computer simulations and theoretical analysis of vibrational phas... more We present results of extensive computer simulations and theoretical analysis of vibrational phase relaxation of a nitrogen molecule along the critical isochore and also along the gas-liquid coexistence. The simulation includes all the different contributions atom-atom AA, vibration-rotation VR, and resonant transfer and their cross-correlations. Following Everitt and Skinner, we have included the vibrational coordinate (q) dependence of the interatomic potential. It is found that the latter makes an important contribution. The simulated results are in good agreement with the experiments. Dephasing time (v) and the root mean square frequency fluctuation in the supercritical region are calculated. The principal important results are: a a crossover from a Lorentzian-type to a Gaussian lineshape is observed as the critical point is approached along the isochore from above, b the root mean square frequency fluctuation shows nonmonotonic dependence on the temperature along critical isochore, c along the coexistence line and the critical isochore the temperature dependent linewidth shows a divergence-like-shape behavior, and d the value of the critical exponents along the coexistence and along the isochore are obtained by fitting. It is found that the linewidths directly proportional to the rate of vibrational phase relaxation calculated from the time integral of the normal coordinate time correlation function C Q (t) are in good agreement with the known experimental results. The origin of the anomalous temperature dependence of linewidth can be traced to simultaneous occurrence of several factors, i the enhancement of negative cross-correlations between AA and VR contributions and ii the large density fluctuations as the critical point CP is approached. The former makes the decay faster so that local density fluctuations are probed on a femtosecond time scale. The reason for the negative cross-correlation between AA and VR is explored in detail. A mode coupling theory MCT analysis shows the slow decay of the enhanced density fluctuations near critical point. The MCT analysis demonstrates that the large enhancement of VR coupling near CP arises from the non-Gaussian behavior of density fluctuation and this enters through a nonzero value of the triplet direct correlation function.