Speed dependence of rotational relaxation induced by foreign gas collisions: Studies on CH3F by millimeter wave coherent transients

Abstract

The dependence of rotational relaxation rates on the speed of absorbing molecules has been studied by millimeter wave coherent transients for the J,K=1,1–2,1 rotational transition of methyl fluoride (CH3F). A new phenomenological model used to describe such a speed dependence has been introduced. It leads to a quite simple analytical expression for time-domain transient signals, the Fourier transform of which corresponds to the frequency-domain line shape (speed-dependent Voigt profile). The investigations were carried out on mixtures of CH3F with He, Ar, Xe, H2, D2, N2, and O2, yielding parameters which characterize the speed dependence of the observed decay rates and its pressure and temperature dependence. Special emphasis was given to the key role of the mass ratio of collision partners which clearly allowed the relation of the observed nonexponential decay behavior to collisional effects. However, the observations cannot be explained exclusively with consideration of speed-dependent rates, but must also be discussed with reference to velocity-changing collisions. The observed temperature dependence of the rates may allow discrimination between these two different collisional effects which lead to departures from Voigt profile line shapes.

Absorption-line-shape recovery beyond the detection-bandwidth limit: Application to the precision spectroscopic measurement of the Boltzmann constant

“…A similar equation holds for the Fourier transform of the absorbance A() which can be interpreted as proportional to the correlation function () of the dipole moment induced in the gaseous medium after a laser excitation starting at time  = 0 [12,25,26] …”

“…(5) development. Moreover, () has an analytical form for a number of line profiles such as Voigt, Galatry, Speed-Dependent Voigt and Speed-Dependent Galatry models [12,[26][27][28][29], including the case of frequency modulated spectral profiles [30]. Of course, the result is not restricted to the 2 nd order low pass filters (or to the 1 st order ones by setting Q = 0), and can be generalized to more complicated detection schemes.…”

Speed-Dependent Effects on the 748.8 nm Ne Self-Broadened Line

“…Most experiments on speed-dependent effects performed in recent years dealt with profiles of molecular lines of H2, CH3 F, CO, C 2 H 2 , and HF molecules perturbed by various gases [14][15][16][17][18][19][20][21][22]. For atomic lines the only experiments in which speed-dependent effects were studied are those performed by McCartan and Lwin [8] for Li-Xe, Lewis and his group [9] for Ca.…”

Frequency-comb-calibrated Doppler broadening thermometry

“…For the speed-dependent Voigt profile, we also considered the quadratic approximation (SDVP-Q), resulting from the first- order expansion of the hypergeometric dependence of Eq. (4) [19]. Figure 5 clearly shows the failure of this approximation.…”