How a collision causes misalignment: alignment decay in acetylene 21

Abstract

The effects of acetylene self-collisions on alignment have been studied in detail. Strongly aligned samples of C2H2 were prepared in several rotational states of the 2l level by stimulated Raman pumping. Laserinduced fluorescence verified the initial degree of alignment and was used to follow its decrease via collisions. Measurements of the initially excited alignment agree well with theoretical calculations of that achievable by stimulated Raman pumping, and the decay can be well modeled by a simple kinetic scheme.

Elastic depolarization and polarization transfer in CN(A2Π,v= 4)+Ar collisions

“…[39][40][41] The alignment retention they observed was very similar to that reported here, averaging 40% for a range of ∆j = ±2 transitions studied. 39 The simplest models to fit these polarization ratios are ones in which limited ranges of either ∆m j or classical tilt angle, ' jj θ , are allowed. With measurements limited to purely one tensor moment of the m j distribution there will always be a range of parameters in such a model that will fit the data.…”

“…We also observe a significant [39][40][41] The orientation and alignment elastic depolarization rates were both measured, and found to be essentially equal, and to decline with increasing j. The similar orientation and alignment rate constants are inconsistent with any simple model in which the magnitude of ∆m j in a single collision is restricted, as these will lead to destruction of alignment faster than orientation.…”

“…[36][37][38] There are fewer more recent experimental studies of other species. Notable exceptions are the work of Rudert et al, who studied elastic depolarization and polarization transfer of both orientation and alignment in C 2 H 2 self-collisions, [39][40][41] and the measurement of alignment transfer in self-collisions of N 2 by Sitz and Farrow. 42,43 Further discussion of these experiments is deferred until later in this paper, but we note that these systems differ in being closed shell and therefore proceeding on single…”

Collisional effects on angular momentum orientation in acetylene X̃ 1Σg+ (ν2″=1,j″). II. Disorientation by rotationally elastic and multiple inelastic collisions

“…Measurements of alignment decay show about the same magnitude. 7,19,31 The collisional orientation decay rate is approximately one third of the average rotational depopulation rate constant of ͗k tot ͘ ϭ25Ϯ2 (s Torr) Ϫ1 . 28 Decays of orientation have also been measured at several pressures, varying the delay time between pumping and probing until the Lennard-Jones collision probability reaches 50%.…”

“…Previously we had discussed the case of elastic transitions that change the alignment in terms of a model where ⌬m j ϭϮ1 which fit the limited data available at that time. 7 Based on a potential energy surface used to calculate accurate rotational energy transfer cross sections for H 2 (B 1 ⌺ u ϩ ) ϩHe, Odutola et al were able to reproduce the experimental data for collisional changes in the molecular orientation. 20,21 A very accurate potential energy surface is now available for C 2 H 2 ϩHe.…”