Rotational Pattern Difference in Resolved Fluorescence Spectra with Different Detection Schemes Hongmin Chen,* ,1 Li Li,* G. Lazarov,† X. Wang,† A. M. Lyyra,† J. Huennekens,‡ ,2 and R. W. Field§ ,3 *Department of Physics, Tsinghua University, Beijing 100084, China; †Department of Physics, Temple University, Philadelphia, Pennsylvania 19122; ‡Department of Physics, Lehigh University, 16 Memorial Drive East, Bethlehem, Pennsylvania 18015; and §Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 E-mail: jph7@lehigh.edu Received June 1, 1998; in revised form March 31, 1999 The relative intensities of rotational lines in resolved fluorescence spectra are dependent on the detection direction and the choice of the detection scheme when a grating monochromator is used. These differences arise from the spatially anisotropic distribution of the fluorescence, the rotational branch dependence of the fluorescence polarization, and the polarization dependence of the monochromator grating efficiency. Both the anisotropy of the emission and the rotational branch dependence of the fluorescence polarization are enhanced in double-resonance excitation schemes. In the present work, we analyze the relative intensities in the 7 Li 2 1 3  g - 3 1( b) 3  u and 1 3  g 3 1( b) 3  u resolved fluorescence spectra, observed following double-resonance excitation, for three different detection schemes. © 1999 Academic Press Key Words: fluorescence detection; spectral patterns of fluorescence; 7 Li 2 -resolved fluorescence; polarization of fluorescence. I. INTRODUCTION Laser-induced fluorescence (LIF) has become a classic tech- nique of laser spectroscopy. LIF spectra are recorded in two formats, fluorescence excitation and resolved fluorescence. A fluorescence excitation spectrum is recorded by monitoring total fluorescence while the laser frequency is scanned. Re- solved fluorescence spectroscopy involves scanning the mono- chromator while holding the laser frequency fixed to a selected transition. Resolved fluorescence spectra provide abundant information about the upper and lower states. Relative intensities of transitions into different vibrational levels of the lower state give information about Franck–Condon factors and transition dipole moments. The rotational structure of a transition (spectral patterns and relative intensities of rotational lines), from a single upper rovibronic level to a lower vibrational level, is often used to determine the sym- metries of the upper and lower states. In resolved fluorescence experiments, the laser-induced flu- orescence can be collected with several different detection schemes. The relative intensities of rotational lines in the resolved fluorescence spectra, however, may depend on the particular scheme chosen. For experiments utilizing double- resonance excitation, the polarization behavior is much more complex than the well-understood and frequently treated one photon excitation case. Here we show examples and quantita- tively explain the differences in the relative rotational branch intensities for perturbation-facilitated optical– optical double- resonance (PFOODR) resolved fluorescence spectra of 7 Li 2 , as studied using three of the most common geometries. II. OBSERVATIONS The experimental setup has been described in Refs. (1) and (2). Lithium vapor was generated in a five-arm heatpipe oven [Fig. 1a]. Two continuous-wave (CW) single-mode, frequen- cy-stabilized CR 699-29 dye lasers were used as the PUMP and PROBE lasers to excite 1 3  g - ( v = 0, N = 9, J = 10) 4 b 3  u ( v '= 19, N'= 10, J '= 11) 4 X 1  g + ( v = 4, J  = 10), 1 3  g ( v = 9, N = 9, J = 10) 4 b 3  u ( v '= 19, N'= 10, J '= 11) 4 X 1  g + ( v = 1, J = 10), or 1 3  g ( v = 17, N = 9, J = 10) 4 b 3  u ( v '= 19, N'= 10, J '= 11) 4 X 1  g + ( v = 1, J = 10) transitions of 7 Li 2 . The b 3  u ( v '= 19, N'= 10, J '= 11) intermediate level is perturbed by the A 1  u + ( v '= 13, J '= 11) level and therefore acts as a window level through which the dark triplet states can be viewed. Both lasers were linearly polarized along the same direction and co- or counterpropagated coaxially. OODR flu- orescence from the upper 1 3  g - or 1 3  g level excited by the lasers, to the b 3  u state, was resolved with a Spex 0.85 m double-grating monochromator. The relative detection system efficiency, as a function of wavelength, was measured using a calibrated tungsten– halogen lamp (“white light source”) (3). 1 Present address: Department of Physics, University of Connecticut, U-46, Storrs, CT 06269. 2 To whom correspondence should be addressed. 3 Also at Department of Physics, Tsinghua University, Beijing 100084 China. Journal of Molecular Spectroscopy 196, 197–211 (1999) Article ID jmsp.1999.7873, available online at http://www.idealibrary.com on 197 0022-2852/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.