Vibration, rotation, and parity specific predissociation dynamics in asymmetric OH stretch excited ArH2O: A half collision study of resonant V–V energy transfer in a weakly bound complex
Authors: Nesbitt David J. , Lascola Robert 
Published: July 2002

Para ArH2O complexes are detected via slit jet direct absorption spectroscopy using continuous wave difference frequency generation of high resolution tunable IR in the 3780 cm−1 v3=1←0 asymmetric OH stretch region. P/R and Q branch transitions on the Π(101,v3=1)←Σ(000,v3=0) band are observed, which derive oscillator strength from the 101←000 v3=1←0 transition in free H2O and access upper states of e and f parity, respectively. Frequency shifts in the f parity levels (Q branch), and both frequency shifts and predissociation broadened linewidths in the e parity levels (P/R branch) are evidenced in the spectrum, unambiguously characteristic of an avoided crossing in the vicinity of J′=6 with a second, near resonant vibration–internal rotation state in ArH2O. From detailed analysis of the avoided crossing, this perturbing state is assigned to a symmetric (v1=1) OH stretch+internal rotor state in ArH2O, which resonantly mixes with the IR laser prepared asymmetric (v3=1) OH stretch by resonant ‘‘half collisional’’ V→V transfer with the Ar atom. The striking parity dependence of the observed predissociation linewidths results from energy conservation and the symmetries of the asymptotic Ar+H2O (v1=1) states; the e parity levels can dissociate to the ground 000 rotor level of H2O (v1=1), whereas the f parity levels can access at lowest the 111 rotor level, therefore releasing insufficient energy to rupture the van der Waals bond.
The data establishes a clear upper limit on the dissociation bond strength for para ArH2O, and place spectroscopic constraints on the promoting internal rotor state in ArH2O (v1=1). A simple local mode theory of half collision induced mixing between symmetric and asymmetric stretch excitation in H2O is presented. The model predicts a coupling which is proportional to three factors, (i) a ‘‘steric’’ term which describes the overlap of initial/final internal rotor wave functions in the ArH2O angular geometry conductive to energy transfer, (ii) a ‘‘collisional’’ term which depends on the van der Waals stretching frequency and reduced mass, and (iii) an intramolecular V→V term to describe the ‘‘rephasing’’ of the OH local modes that characterizes the collisional transformation between asymmetric and symmetric stretch vibrations. All the parameters in the model are accessible from the high resolution van der Waals spectra, and the resulting predictions are in surprisingly good agreement with the coupling spectroscopically determined from the perturbation analysis. The high efficiency of the predissociation dynamics is also qualitatively consistent with the near gas kinetic v3↔v1 energy transfer observed by Finzi et al. [J. Chem. Phys. 63, 2285 (1975)] in thermally averaged H2O(v1,v3)+Ar collisions. In summary, analysis of the J and parity dependent predissociation linewidths and frequency shifts in a van der Waals near-IR spectrum of ArH2O offers an unusually detailed glimpse at collision induced intramolecular V–V transfer in vibrationally excited H2O from a half collision perspective.