Messenger spectroscopy

As mentioned in Section 9.2, there is an important distinction between action spectroscopies based on the absorption of multiple photons, as is possible with FEL excitation, and those based on single photon absorption, such as messenger spectroscopy [26,27], laser-induced reaction spectroscopy [73] or indeed direct absorption experiments. When the signal (in this case, the depletion of the parent ion and creation of product ious) depends on the absorption of multiple photons, the signal intensity may not be compared directly to computed linear (single-photon) absorption spectra, as is the normal fashion. There are several reasons why intensities may differ. Thus, baudpositiou (frequency) is considered usually to be more reliable than band intensity when comparing computed and experimentally-measured spectra. 

Various groups have now implemented messenger spectroscopy in different forms using a variation of MS hardware. Instruments based on triple-quadrupole (-like) geometries were developed [117, 120-122] (see Fig. 4) and have, among others, led to the successful application of messenger spectroscopy in cryogenic ion... 

Fig. 4 Typical tandem MS set-up for messenger spectroscopy based on quadrupole mass selector and analyser tuned to the m/z values of the weakly bound ion-ligand complex (Alf -L) and that of the bare ion (AfT"), respectively. The ligand is usually a small molecule (e.g. H2) or a rare gas atom. Figure adapted from Dopfer [117]...

By forming ion-rare gas atom (Rg) complexes, the dissociation threshold of the system is lowered, generally below the photon energy and these predissociation spectra directly reflect the linear absorption spectrum. This technique has also been used to great effect in anion spectroscopy experiments [13]. The multiphoton dissociation approach remains attractive for systems in which the perturbation of the messenger atom carmot be neglected, or in instruments where rare gas attachment is difficult. 

Laser-Induced Spectroscopy of a Messenger-Tagged Ionic Cluster... 

C. Arpigny, F. Dossin, J. Manfroid, P. Magain, A.C. Danks, D.L. Lambert, C. Sterken Spectroscopy, photometry and direct filter imagery of comet P/Halley. ESO Messenger 45, 10 (1986)... 

An inverted version of the messenger tagging technique for detecting ion absorption uses the fact that electronic and/or vibrational excitation of ions hinders formation of weakly-bound clusters. This effect, explored years ago in relation to laser isotope separation [82], has recently been demonstrated for spectroscopy of N2" ions, cooled to 10.6 K in a 22-pole trap by collisions with He and termed laser-induced inhibition of cluster growth (LIICG) [83]. An electronic spectrum is generated by monitoring the reduction of the steady-state concentration of ion-He complexes as a function of the excitation laser wavenumber. 

Protopapa, S., Herbst, X, Bohnhardt, H. Surface ice spectroscopy of Pluto, Charon and Triton. Messenger 129, 58-60 (2007)... 

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