High resolution ionization-detected raman gain spectroscopy of N-2 and C6H6

Double resonance spectra have been obtained for nitrogen and benzene molecules in the low density region of a molecular beam by use of stimulated Raman pumping followed by ionization probing. For N-2 the photoionization step was via a 2+2 resonantly enhanced multiphoton ionization process using the a(1) Pi(g)(v = 2)<--X(1) Sigma(g)(v = 1) two-photon resonance. The detection limit is 10(3)-10(4) times lower than for the direct optical measurement of a stimulated Raman signal. For benzene, a more efficient 1+1 ionization step permitted measurements at state densities less than 10(10) cm(-3) and the Raman step yielded linewidths of 30 MHz for the Q-branch lines of the weak Raman active nu(16) fundamental. (C) 1996 American Institute of Physics.