Preferential stabilization of different isomers of weakly bound complexes

The experimental results reported here demonstrate that weakly bound van der Waals complexes are not necessarily kinetically trapped in different isomeric forms in a supersonic expansion. We have recorded the laser-induced fluorescence spectra of the near T-shaped and linear He...ICl(X(1)Sigma(+),nu" = 0) complexes in the ICl B-X, 3-0 spectral region at varying distances along the expansion direction to monitor the populations of the two isomers as the local density, collision frequency, and temperature change. The ratio of the T-shaped to linear complex populations monotonically decreases with increasing downstream distances and thus lower ICl(X) rotational temperatures. Conversion from T-shaped to linear complexes is still observed at the furthest distances where a temperature of 0.57(8) K is measured.