The femtosecond birefringence of CO2: from the high pressure gas to the liquid phase

Abstract

The optical heterodyne detected (OHD) birefringence of CO2 gas in the reduced density range from ρ/ρc = 0.09–0.82 and in the room temperature liquid are reported. Each birefringent response is separated into an electronic, rotational diffusion and non‐diffusive nuclear component. The CO2 rotational relaxation times in the gas phase scale inversely with density in accordance with extended diffusion models of rotational reorientation. The rotational diffusion component of CO2 liquid (τD = 0.34 ps) appears to obey Debye–Stokes–Einstein behavior. The non‐diffusive nuclear responses of the vapor samples exhibit only very modest changes with density and closely resemble the response function of the classical linear free rotor. The dominant, but not exclusive, contribution to the decay of the Raman anisotropy of high‐pressure CO2 gas is inertial motion. The spectral density of the non‐diffusive nuclear response of liquid CO2 is broader and has higher frequency components than the corresponding gas‐phase spectral densities, indicating the relative importance of interaction induced effects. The greater extent of collision‐induced effects is clearly evident for CS2 liquid compared with liquid CO2 when the spectral densities of the non‐diffusive nuclear responses of liquid CO2 and CS2 are compared as a function of reduced frequency. Inertial motion clearly makes a much larger contribution to the birefringence of liquid CO2 than to that of liquid CS2 (at room temperature). Copyright © 2000 John Wiley & Sons, Ltd.

Ultrafast H2 and D2 rotational Raman responses in near critical CO2: An experimental and theoretical study of anisotropic solvation dynamics

“…The low-frequency region ͑Ͻ200 cm −1 ͒ of the Raman spectral density is dominated by the nuclear motions of the near critical CO 2 and can be described in terms of diffusive and nondiffusive contributions to the anisotropic orientational relaxation. 22 The ϳ20 fs pulses employed in this OHD study allow quantitative analysis of the S͑0͒, S͑1͒, and S͑2͒ transitions of H 2 at 356.1, 587.4, and 813.5 cm −1 , respectively, and the S͑0͒, S͑1͒, S͑2͒, and S͑3͒ transitions of D 2 observed at 182.8, 298.7, 413.7, and 528.8 cm −1 correspondingly. The J-specific H 2 ͑D 2 ͒ ͉⌬J͉ = 2 rotational Raman transitions appear as discrete bands in these spectral densities with J-dependent ͑FWHM͒ bandwidths and slightly asymmetric line shapes ͓Figs.…”

“…21,22 The / 2 out-of-phase local oscillator field for optical heterodyne detection was generated by a small rotation of a polarizer in front of a quarter wave plate in the probe arm. Near transform-limited ϳ20 fs Ti:sapphire oscillator pulses at 76 MHz were used to obtain the reported rotational Raman responses.…”

“…Femtosecond impulsive Raman spectroscopy in the standard two-pulse OHD experimental configuration 21,22 is ideally suited for observing the pure Raman rotational ͑⌬J = Ϯ 2͒ transitions of H 2 / D 2 whose cross sections are given by the anisotropic part of the Raman scattering tensor. The spontaneous rotational Raman scattering of H 2 in supercritical CO 2 has been reported in one prior experimental study.…”