RESONANTLY ENHANCED MULTIPHOTON IONIZATION OF N2 A''1-SIGMA-G+ (UPSILON' = UPSILON'') [- X1-SIGMA-G+ (UPSILON'' = 0-2) .1. DETERMINATION OF ROTATIONAL POPULATIONS AND VIRTUAL STATE CHARACTER

The resonantly enhanced 2 + 1 multiphoton ionization spectrum (REMPI) of the (0,0), (1,1), and (2,2) transitions of the a"1-SIGMA(g)+ <-- X1-SIGMA(g)+ states of molecular nitrogen has been recorded at high resolution, and the rotational and vibrational constants of the excited states have been calculated. The sensitivity of the REMPI technique is 5 x 10(5) molecules/(cm3 quantum state). The (0,0) transition was originally seen by Lyke and Kay in REMPI and by Dressler and Lutz in one-photon emission spectroscopy. Our measured spectroscopic constants are as follows (in cm-1): T(e) = 98 938 +/- 5, omega'(e) = 2167 +/- 3, B'(e) = 1.938 +/- 0.001, alpha'e = 0.047 +/- 0.001, nu-00 = 98 840.59 +/- 0.12, B'upsilon=0 = 1.9143 +/- 0.0002, D'upsilon=0 = 6.6 x 10(-6) +/- 0.2 x 10(-6), upsilon-11 = 98 655.3 +/- 0.2, B'upsilon=1 = 1.882 +/- 0.001, upsilon-22 = 98 458.8 +/- 0.2, and B'upsilon=2 = 1.820 +/- 0.001. In addition, some general questions concerning 2 + n REMPI SIGMA <-- SIGMA-transitions have been addressed by using N2 a"1-SIGMA(g)+ <-- 1X-SIGMA(g)+ as a model. In two-photon SIGMA <-- SIGMA-absorption spectroscopy, the relative intensities of the different rotational branches (O, Q, and S) and the polarization line strengths of the Q branch depend very strongly upon the SIGMA versus PI-character of the virtual state. Accurate measurement of the character of the virtual state was made using two methods: recording the relative two-photon absorption intensity of the O- and Q-branch lines and recording the two-photon absorption intensity as a function of the elliptical polarization of the laser light. For nitrogen, from measurement of the relative intensities of the O and Q branch, we have determined the virtual state path ratio: mu(i)2/mu(s)2 = 12.5 +/- 5, where the path ratio is a function of the SIGMA versus PI-character of the virtual state. While the Q-branch measurements are always sensitive to the rotational populations and quadrupole alignments moments, measurements of absorption via the S and O branches are critical to determining the angular momentum orientation of an anisotropic sample. Therefore, accurate assignment of O- and S-branch lines and determination of the character of the virtual state are critical to using any two-photon SIGMA <-- SIGMA-transition for determination of populations, alignment, and orientation.