IDSRS

Plenary 9. J W Nibler et al, e-mail address niblerj chem.orst.edu (CARS and SRS). High resolution studies of high lymg vibration-rotational transitions in molecules excited in electrical discharges and low density monomers and clusters in free jet expansions. Ionization detected (REMPI) SRS or IDSRS. Detect Raman... 

IDSRS) and the complementary resonance ion-dip IR spectroscopy (RID1RS) are useful mass selective techniques for the probing of cluster ground state vibrations. 

Figure 3.6-5 Energy-level diagram illustrating the two excitation steps of Ionization Detected Stimulated Raman Spectroscopy (IDSRS).

S Ionization detected stimulated Raman spectroscopy (IDSRS)... 

Since IDSRS involves the detection of ions, it can be readily implemented with mass-selective detection of those ions (Henson et al., 1989 Hartland et al., 1990 Henson et al., 1991). In fact, ion-mass analysis (by time-of-flight techniques, for example) greatly enhances the capabilities of the IDSRS technique. 

Besides the frequency domain versions of mass-selective IDSRS, in which the frequency is scanned while photo-ions are detected, the same group has also developed Fourier transform versions of this scheme (FT-IDSRS) (Hartland et al., 1990). In this method the two stimulating Raman excitation beams are directed through a Michelson interferometer before being combined with the ionization UV laser field (see Fig. 3.6.-18). 

Photo-ions are then detected as a function of interferometer delay. The result is an in-terferogram that upon Fourier transformation yields a Raman spectrum whose resolution does not depend on the bandwidths of the Raman excitation sources but, instead, on the delay range of the interferometer scanned in the experiment. Mass-selective IDSRS and FT-IDSRS have been employed in a number of studies, including one of the benzene dimer (Henson et al., 1991). For examples of IDSRS spectra see Sec. 6.1.4.5. 

Structure of the vibrational 0-branch was resolved by pulsed CARS (Beckmann et al., 1981) and by IDSRS (ionization-detected stimulated Raman scattering, Esherick and Owyoung, 1983). 

Figure 6.1-29 Ionization detected stimulated Raman (IDSRS) spectra of benzene in the region of overlap between the "0-branch transitions of and the S-branch transitions of V2 + v ,. (a) UV laser tuned to 36 467 cm (b) UV laser tuned to 36 496 cm (Esherick et al 1985).

IDSRS spectra of the benzene dimers C6H6 - (which are denoted do - d(,) and CfeDf, - CftD6 (dfi - de), respectively. From measurements of a number of isotopomers of the species in several spectral regions of the benzene monomer, several significant results relating to structure have been obtained (Felker et al., 1992). 

From the sophisticated measurements performed in Felker s laboratory it seems that the various versions of ionization-detected. stimulated Raman techniques (with mass analysis) have great capabilities in the high resolution vibrational spectroscopy of weakly bound complexes and clusters. One expects that IDSRS will become increasingly productive in the studies of cluster ground-state structure and dynamics. 

All participants in an initialization must input the same value as idsj idsR initR, and idsR i Rc, respectively. It will be called idsRj u. 

Fig. 7 Raman gain = stimulated Raman gain spectroscopy (SRGS), inverse Raman = inverse Raman spectroscopy (IRS) or stimulated Raman loss spectroscopy (SRLS), coherent anti-Stokes Raman spectroscopy (CARS), photoacoustic Raman spectroscopy (PARS), or ionization-detected stimulated Raman spectroscopy (IDSRS). In the following sections, the various methods are briefly described. More detailed information can be found in books [59-61], reviews [45,46,57,58,62,63] and conference reports [64-73].

Figure 11 Energy-level diagram for IDSRS processes. a) and b) are ground-state vibrational levels, e) is an excited electronic state level, O is the Raman transition frequency between a) and b), and CO2, and (O3 are the pump, Stokes, and probe laser frequencies, respectively. (From Ref. 112, with permission.)...

The unpaired electron in the molecule NO gives rise to two states 113/2 and A Raman transition between these states is allowed and was detected by Rasetti [8] at about 120 cm This electronic Raman effect was rotationally resolved by Fast et al. [22] and Rich and Welsh [160]. The rotational structure of the vibrational Q branch was resolved by pulsed CARS [161] and by IDSRS [108]. Recently, Raman spectra of the dimer (NO)2 were recorded in a molecular beam [162]. 

A technique which combines the high sensitivity of resonant laser ionization methods with the advantages of nonlinear coherent Raman spectroscopy is called IDSRS (ionization detected stimulated Raman spectroscopy). The excitation process, illustrated in Figure 5, can be briefly described as a two-step photoexcitation process followed by ion/electron detection. In the first step two intense narrow-band lasers (ct L, 0) ) are used to vibrationally excite the molecule via the stimulated Raman process. The excited molecules are then selectively ionized in a second step via a two- or multiphoton process. If there are intermediate resonant states involved (as state c in Figure 5), the method is called REMPI (resonance enhanced multi-photon ionization)-detected stimulated Raman spectroscopy. The technique allows an increase in sensitivity of over three orders of magnitude because ions can be detected with much higher sensitivity than photons. 

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