Rotational energy transfer in vibrationally excited acetylene X̃ 1Σg(ν2″=1,J″):ΔJ propensities

Abstract

A complete set of state-to-state rotational energy transfer rate constants has been measured for acetylene–acetylene collisions at room temperature under single collision conditions. Initial rotational states (Ji=5,7,...,25) were prepared and final states (Jf=1,...,25) interrogated. The measurements were carried out in a typical gas phase pump and probe arrangement. The initial vibrationally excited state was prepared by stimulated Raman pumping using strong Q-branch transitions. State preparation via this branch produces an isotropic spatial distribution of the excited state which is important for data analysis. Narrow bandwidth lasers ensure single rotational state selectivity. The rotational distribution after collisions is monitored by time-delayed laser-induced fluorescence via the à 1Au(ν3′ = 1)←X̃ 1Σg(ν2″ = 1) transition. In general, the rate constants decrease exponentially with the transferred rotational energy. The complete rate constant matrix can consistently be described by a simple parameter set within the dynamical infinite order sudden power approximation. In addition to this general behavior a significant ΔJ propensity of the rate constants is observed. Using the energy corrected sudden approximation with a power law basis an excellent match, reproducing the ΔJ propensities, to the rate constant matrix is obtained, again with a single set of parameters.

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

“…Using state-to-state rotational energy transfer constants k j Ј ← j taken from Ref. 28 it can be calculated that after this delay 79% of the molecules reaching j f Љϭ9 have collided only once, but 21% of the molecules reaching this state have undergone two or more collisions.…”

“…1 so that we can compare these measurements with our previous work on population decay. 28 From Part I ͑Ref. 1͒ the average LIF intensity can be expressed as Ī(t)ϭn( jЉ,t)͓a…”

“…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%. Figure 4 is a typical Stern-Vollmer plot of the decay rates k ori total •p shown here for jЉϭ9 after preparation via the S(7) transition.…”

“…One first calculates the fractional amount of population ␦n( jЈ, j) transferred between all sets of states in the time interval ͓t,tϩ⌬t͔ by using the rate constants k j Ј ← j taken from Ref. 28. The orientation of these packets is calculated from the orientation in state j at time t using from Part I ͑Ref.…”

Rotational and vibrational energy transfer in vibrationally excited acetylene at energies near 6560 cm−1

“…The PEGL parameter values and the rate constant matrices reconstructed using Eqs. (8) and (9) are presented in Tables II and IV for the (0,1,1,2 0 ,0 0 ) and (1,0,1,0 0 ,0 0 ) states, respectively.…”