Observation of collisional relaxation from HD v=5 and v=6 by direct overtone pumping and photoacoustic detection

We present here the first detailed measurement on the mechanism of collisional relaxation from the upper vibrational states of HD. Preliminary results are given in which we have measured collisional relaxation following the direct overtone pumping of HD into v=5 and v=6 with a pulsed visible wavelength dye laser. These time-resolved photoacoustic data indicate that the vibrational to transitional (V-T) and vibrational to vibrational (V-V) relaxation rates for collisions with ground state molecules become equal near v=5.