Collisional Interference Research Articles

Broadening and collisional interference of lines in the IR spectra of ammonia: Self-broadening in the ν1 band

Broadening and collisional interference of lines in the IR spectra of ammonia: Self-broadening in the ν1 band

Broadening and collisional interference of lines in the IR spectra of ammonia. Theory

The general theory of relaxation spectral shape parameters in the impact approximation (M. R. Cherkasov, J. Quant. Spectrosc. Radiat. Transfer 141, 73 (2014)) is adapted to the case of line broadening of infrared spectra of ammonia. Specific features of line broadening of parallel and perpendicular bands are discussed. It is shown that in both cases the spectrum consists of independently broadened singlets and doublets; however, the components of doublets can be affected by collisional interference. The paper is the first part of a cycle of studies devoted to the problems of spectral line broadening of ammonia.

Broadening and collisional interference of lines in the ir spectra of ammonia: Self-broadening in the ν2 band

The relaxation parameters of the lines of the P, Q, and R branches of the ν1 ammonia ν1 band are calculated in the case of self-broadening. In the case of doublets, the effects of collisional interference of the doublet components have been taken into account. It is shown that the cross-relaxation parameters of the components can reach ~60% of the values of the self-broadening coefficients, which gives rise to the narrowing of the components and indicates that the isolated line approximation is inapplicable. Comparison with experimental data is made. Good agreement for self-broadening coefficients is obtained. In the case of the self-shift coefficients, discrepancies are considerable in both magnitude and sign, and it is impossible to elucidate their reasons without invoking additional experimental data.

Statistical models of scalar collisional interference incorporating phase shifting

In this paper we include phase shifting in the simple statistical models of collision sequence interference developed earlier by the authors. The Lindholm model of shifting and broadening is used.

Pressure-induced broadening of hyperfine components of rotational transitions of methyl cyanide molecule

The broadening of hyperfine components of rotational transitions of the methyl cyanide molecule is considered in terms of the collision approximation based on the theory previously developed by the author. The role played by possible collisional interference processes is examined. Relaxation parameters are calculated, and broadening coefficients are compared to the experimental data.

Effects of collisional interference of lines in the spectra of symmetric top molecules: III. Broadening of rotational transitions with a hyperfine structure

The peculiarities of the broadening of rotational transitions of molecules in the case where the spectrum exhibits a hyperfine structure caused by the nuclear quadrupole interaction are considered. The relaxation parameters of the components of a number of hyperfine multiplets of CH3I molecules are calculated taking into account self-broadening, and calculations are compared with experiment.

Effects of collisional interference of lines in the spectra of symmetric top molecules: II. Self- and foreign-gas-broadening of rotational spectral lines

The theory outlined in the first part of the study is applied to an analysis of self-broadening and broadening by foreign gases of spectral lines for a number of symmetric top molecules with a high potential barrier to the inversion vibration. In all cases considered, the results obtained are in better agreement with the experimental data than previous theoretical results; however, in some cases, the discrepancy is rather large. Possible reasons are discussed and the acute need for experimental data obtained with modern equipment is noted.

Collisional interference of lines in the spectra of symmetric top molecules: I. A theory of relaxation parameters of the shape of the spectrum in the impact approximation

The peculiarities of broadening of spectral lines of symmetric top molecules that are related to the double degeneracy of JK levels at K ≠ 0 are considered. It is shown that, irrespective of the potential barrier height for the inversion vibration, transitions between such levels should be considered as doublets whose components are coupled with each other by the collision-induced spectral exchange. The shape of the doublet is described by the well-known Ben-Reuven formula, and, in this case of molecules with a high potential barrier for the inversion vibration, is a dispersion contour with a half-width equal to the difference between the halfwidth of the component of the doublet and the cross-relaxation parameter.

Collisional interference in the infrared spectrum of HD: calculation of the line shape of vibrorotational transitions for HD-He

The line shape parameters for the collision-induced vibration-rotation spectrum of HD-He are calculated with the formalism of Gao et al. The total intensity interference parameters and line asymmetry parameters are found to be larger than those for pure rotational transitions. The results depend on collisional propagation among rotational levels in both vibrational states and can be sensitive to vibrational-state dependence of the intermolecular potential.

Role of the induced dipole moment in the collisional interference in the pure rotational spectrum of HD-He and HD-Ar.

The parameters describing the interference between allowed and collision-induced transitions in the far-infrared spectrum of HD-He and HD-Ar are computed following the formalism of Gao et al. [Phys. Rev. A 44, 7379 (1991)] and the calculational procedure of Gao et al. [Phys. Rev. A 46, 5781 (1992)]. The contribution from each component of the induced-dipole moment is identified. The interference between the induced-dipole component having the same symmetry as the allowed moment and the allowed moment itself has the largest single contribution to the intensity interference effect. The mixing of rotational levels by an anisotropic intermolecular interaction introduces other components into the interference mechanism. The overall effect is usually to decrease the magnitude of the interference and to permit line broadening and asymmetry to occur. This interaction is most effective in causing the anisotropic overlap-induced pair dipole moment ${\mathit{B}}_{2}$(21) to play a role in the interference mechanism. The effect of the isotropic overlap-induced dipole moment is usually negligible.

Kinetics of Thermal Unimolecular Dissociation by Ambient Infrared Radiation

The truncated Boltzmann distribution is used for interpreting the Arrhenius activation energy to obtain an estimate of the ion dissociation energy. This is used to show that the experimental observation of infrared radiation-induced thermal dissociation of ions on a time scale is inevitable at pressures below the limit of collisional interference. 25 refs., 10 figs.

Temperature dependence of the pure rotational band of HD: Interference, widths, and shifts.

The far-infrared pure rotational absorption spectra of gaseous HD and HD-X (X=${\mathrm{H}}_{2}$, He, Ne, Ar, Kr, and ${\mathrm{N}}_{2}$) were measured at 195 and 296 K. Values of the allowed dipole-moment matrix elements, the absolute frequencies, the spectral-line-shape parameters (broadening and frequency-shift coefficients), and the interference parameters, for the first four rotational lines, were deduced from the spectra. Theoretical calculations of the interference parameter a and the linewidths were performed, based, respectively, on the theory developed by Herman, Tipping, and Poll [Phys. Rev. A 20, 2006 (1979)] and a semiclassical theory developed by Robert and Bonamy [D. Robert and J. Bonamy, J. Phys. 40, 923 (1979)]. The induced dipole moments for HD-Ne and HD-${\mathrm{N}}_{2}$ were estimated. The analysis includes the present experimental results, together with previous determinations at 77 and 296 K [L. Ulivi, Z. Lu, and G. C. Tabisz, Phys. Rev. A 40, 642 (1989); P. Drakopoulos and G. C. Tabisz, ibid. $bold 36 ---, 5556 (1987)]. The behavior of the temperature dependence of the collisional interference in the pure rotational band for HD and its mixtures is confirmed to be more complicated than predictions of the intracollisional theory.

Calculations on laser-induced collisional energy transfer.

We examine theoretically the behavior of cross sections for laser-induced collisional energy transfer (LICET) over a wide range of detunings and laser intensities for the two-atom pairs Li-Sr and Eu-Sr which have been previously studied experimentally. These two cases are complementary in that the laser and collisional interactions enter in opposite orders, and their level structures are such as to give weak-field line shapes of opposite asymmetry and intensity shift. A three-state model is used and the Bambini-Berman formula for the weak-field quasistatic wing is verified. At high intensities the peak cross section is found to go through a maximum and the LICET profile is found to become symmetric about a shifted center with notable departures from a uniform falloff in the far quasistatic wing, apparently arising from radiative collisional interference effects.

Spectral line shape arising from collisional interference between electric-dipole-allowed and collision-induced transitions.

A theory is developed to describe the spectral line shape due to interference between electric-dipole-allowed and collision-induced transitions in pure rotational molecular spectra. Motivation was provided by experimental data available for HD-inert gas systems. This theory is based on a master-equation approach to induced spectra employed by Alber and Cooper [Phys. Rev. A 33, 3084 (1986)]. The active molecule is considered to be immersed in a bath of perturbers. An expression for the absorption coefficient is obtained within the binary collision approximation that contains terms due to allowed, induced, and interference contributions. Effects due to m mixing, J mixing, and successive collisions are included. Low-order approximations of the theory eventually reduce to results of earlier efforts, namely, the pioneering description of collisional interference by Herman, Tipping, and Poll [Phys. Rev A 20, 2006 (1979)] and refinements to it through consideration of rotational level mixing. The principal attribute of this approach is the treatment of allowed and collision-induced transitions in a consistent manner.

Decoupling in the line mixing of acetylene infrared Q branches

The Q-branch profiles of the ν1 +ν5 , ν3 +ν4 and ν2 +2ν4 +ν5 Πu –Σg combination bands in the 2.5 μm C–H stretch-bend region of acetylene have been recorded with a difference-frequency laser spectrometer at pressures from 1 to 500 Torr (0.13 to 66.7 kPa). The broadening coefficients, obtained from the ν1 +ν5 band at pressures low enough to avoid significant spectral overlap, can be well fit with empirical rotationally inelastic energy-gap scaling laws or satisfactorily modeled with semiclassical line broadening theory using known intermolecular potential parameters. At pressures when lines are overlapped, collisional interference or line mixing is manifest as a deviation of the Q-branch profiles from an additive superposition of individual transition components. However the line coupling given by the state-to-state collisional scaling laws used to fit the broadening coefficients predicts far more collisional narrowing or Q-branch collapse than is observed. We find that only about one-third of the collisions that broaden the individual lines effectively couple the lines within the f sublevel of the l-doubled excited Π vibrational state observed in the Q branch. This decoupling indicates that there is little or no propensity for preserving the vibrational angular momentum sublevel upon collision, and that elastic reorientational and dephasing collisions may also be-significant. Additionally, we find that the collisional parameters and decoupling are independent of the vibrational state despite dramatically different spectral overlaps exhibited by the three bands studied and a close Fermi resonance between the lower two vibrations. This implies that vibrational relaxation and dephasing collision rates are negligible compared with rotationally inelastic and reorientational rates and usually can be ignored for infrared spectral broadening.

Temperature dependence of the collisional interference in the pure rotational far-infrared spectrum of HD.

The far-infrared pure rotational absorption spectrum of HD and HD-/ital X/ mixtures was measured at 77 and 195 K. Specifically, the rotational lines /ital R/(0)--/ital R/(2) were observed for HD-HD, HD-He, HD-Ne, and HD-H/sub 2/ at 77 K and /ital R/(0)--/ital R/(3) for HD-HD, HD-He, and HD-Ar at 195 K. Refined values of the permanent dipole moment of HD are deduced. Collisional interference in the spectrum is characterized by a parameter proportional to the ratio of the average-induced dipole moment to the permanent dipole moment. Its variation with temperature, particularly a change in sign, is not explainable by current theory. The spectral line-shape parameters of width, frequency shift, and asymmetry are determined and their density and temperature behavior discussed with reference to the impact theory of line broadening.

Collisional interference in the foreign-gas-perturbed far-infrared rotational spectrum of HD.

The rotational transitions R(1), R(2), and R(3) of HD have been studied in absorption with 0.06-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ resolution in mixtures of HD with He, Ne, Ar, Kr, Xe, ${\mathrm{H}}_{2}$, and ${\mathrm{N}}_{2}$ at 295 K and at densities between 3 and 62 amagat. Collisional interference was observed and the interference parameter, proportional to the ratio of the average induced moment to the allowed moment, showed a dependence on system size and ranged from +10\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ ${\mathrm{amagat}}^{\mathrm{\ensuremath{-}}1}$ for light perturbers to -24\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ ${\mathrm{amagat}}^{\mathrm{\ensuremath{-}}1}$ for the heavier ones. These interference effects in the pure rotational band are an order of magnitude smaller than those of the fundamental vibrational band and show a strong J dependence not predicted within current theory. The sign of the isotropic overlap component of the induced moment was determined experimentally in an unambiguous way and its trend was found to be within theoretical expectation; specifically, the sign changes as the mass increases. The line shapes were mostly symmetrical and Lorentzian; asymmetry was observed mainly for the R(1) transition with the low-frequency side of the lines super-Lorentzian and the high-frequency side sub-Lorentzian. Investigation of the effect of collision duration on the autocorrelation function of the allowed moment showed it was not important to the integrated allowed absorption.

Far-infrared rotational spectrum of HD: Line shape, dipole moment, and collisional interference.

The rotational transitions R(0) to R(3) in pure HD gas have been studied in absorption with 0.06-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ resolution at densities from 2 to 68 amagat at 295 K. The permanent dipole moment of HD was found to be 8.83(28), 7.94(2), 7.88(3), and 8.43(10) for R(0) to R(3), respectively, in units of ${10}^{\mathrm{\ensuremath{-}}4}$ D. These are in general agreement with ab initio calculations except for an unpredicted strong rotational state dependence. Collisional interference was observed and the interference parameter, proportional to the ratio of the average induced dipole moment to the allowed moment, is negative for R(0) and R(1), positive for R(2) and R(3), and of a magnitude consistent with calculation. Comparisons are made with earlier work, particularly that of McKellar [Can. J. Phys. 64, 227 (1986)].

Interference effects in the spectrum of HD: IV: the pure rotational band at room temperature

The rotational spectrum of HD has been studied in absorption at room temperature for a density range of 6–57 amagat. Spectra were obtained in the 170- to 360-cm−1 region, including the R0(1), R0(2), and R0(3) transitions, with a 1-m path length and a spectral resolution varying from 0.05 to 0.20 cm−1. The observed line strengths were used to determine values for the dipole transition moments of HD in the range of 7.4 to 7.8 × 10−4 D, which is somewhat lower than currently accepted theoretical values of about 8.3–8.4 × 10−4 D. Only very small effects (≈0.2% per amagat) were found due to collisional interference on the line strengths; this result contrasts with much larger interference effects observed in the fundamental band, and it also casts some doubt on other recent studies of the rotational spectrum where larger interference effects were reported.

Interference effects in the spectrum of HD: III. The pure rotational band at 77 K for HD and HD–Ne mixtures

The pure rotational R(0) transition of HD has been studied in pure HD and in HD–Ne mixtures at a temperature of 77 K and at densities between 3 and 123 amagat. Limited measurements of R(1) in pure HD were also made. A value of 8.18 ± 0.26 D was obtained for the R(0) permanent dipole moment of HD, in reasonable agreement with the theoretical value of 8.3 D. However, no evidence for destructive collisional interference effects at high densities was found, and in fact a slight constructive interference was noted. Thus, these results stand in contrast with those for the fundamental band, and with other recent experiments on the pure rotational band, in which destructive interference was invariably measured.