Preparation and decay of alignment in N2 (v=1)

Abstract

We report measurements of the collisional decay of the angular momentum alignment for N2 (v=1) at 298 K. Stimulated Raman pumping of S-branch (v=1←0) transitions is used to prepare a selected rotational state of N2 with an anisotropic spatial J distribution in the v=1 state. After allowing an appropriate time interval for collisions to occur, 2+2 resonance-enhanced multiphoton ionization is used (through the a 1Πg←X 1Σ+g transition) to detect the relative population and alignment of the pumped level and other levels to which rotational energy transfer has occurred. We have performed a series of measurements in which a selected even rotational level (Ji=0–14) is excited and the time-dependent level population and alignment are measured at several delay times. We find the decay of alignment to be no faster than the decay of the population of the pumped level, indicating that pure m-state changing collisions are slower that J-state changing collisions. We have also observed substantial alignment of molecules which have undergone a J-state changing collision. The alignment is this case is found to be inconsistent with a Δm=0 selection rule often used in models of rotational energy transfer rates.