Vibrational relaxation dynamics of I³⁵Cl(B, ν′) induced by low-temperature collisions with He atoms

Using laser-induced fluorescence and two-laser, pump-probe spectroscopy, collision-induced vibrational relaxation is observed to compete with the dissociation of electronically excited ICl in a helium carrier gas expansion. By thoroughly characterizing the expansion properties, we observe that collisions of ICl(B, ν′ = 3) molecules with He atoms in the expansion induce vibrational relaxation of the initially prepared dihalogen down to rotor states in the next lower ICl(B,ν′ = 2) level on timescales that compete with the rate for non-adiabatic transfer from the B state to the Z1 state. The resulting ICl(B,ν′ = 2,j′) product rotational distribution, along with the analogous ICl(B,ν′ = 1,j′) distribution formed by collisional relaxation of molecules in the long-lived ICl(B,ν′ = 2) level are compared to ICl(B,ν′ = 2,j′) products formed by vibrational predissociation of He⋯ICl complexes prepared in different intermolecular vibrational levels within the He + ICl(B,ν′ = 3) potential. No evidence is observed for resonance-enhanced collisional cross sections, even at the low temperatures achieved, T < 1.0 K.