Direct measurement of rotational energy transfer rate constants for H35Cl (v=1)

Abstract

We investigate the rotational energy transfer of H35Cl (v=1) in collisions with thermal HCl at 298 K. Rotationally state-selective excitation of v=1 over J=1–6 is achieved by stimulated Raman pumping, and the rotational relaxation from the initially pumped level is monitored via 2+1 resonantly enhanced multiphoton ionization (REMPI) through the E 1Σ+–X 1Σ+0–1 band. The ions are detected in a time-of-flight mass spectrometer in order to ensure that only relaxation of H35Cl is observed. We present empirical correction factors for determining relative rotational populations from the REMPI spectral line intensities and extract the rate constants for rotational energy transfer from the time-dependent populations using numerical techniques. The excellent sensitivity of the REMPI technique makes it possible to monitor the relaxation on very short collisional time scales (<0.1 hard sphere collision) and thereby enables us to determine the rate constants for both single quantum and multiquanta rotational transitions directly, without the use of a simplifying model for the matrix of rate constants. The measured rate constants illustrate the importance of multiquanta transitions for HCl; such transitions account for ∼30% of the total population loss from a given rotational level. We compare our rate constants to those obtained in previous measurements on HCl (v=1) and HF (v=2). We find that the rate constants are not adequately described by models based solely on the rotational energy defect of the vibrationally excited molecule.