Three-photon spectroscopy

In a two-photon absorption process the first photon takes the molecule from the initial state 1 to a virtual state V and the second takes it from V to 2. As in Raman spectroscopy, the state V is not an eigenstate of the molecule. The two photons absorbed may be of equal or unequal energies, as shown in Figures 9.27(b) and 9.27(c). It is possible that more than two photons may be absorbed in going from state 1 to 2. Figure 9.27(d) illustrates three-photon absorption. 

Dipole matrix elements, one- vs. three-photon excitation, coherence spectroscopy, 163—166... 

Petkov, M. R Weber, M. H. Lynn, K. G. Rodbell, K. P. 2001. Porosity characterization by beam-based three-photon positron annihilation spectroscopy. Appl. Phys. Lett. 79 3884-3886. 

Laser spectroscopy of the 1S-2S transition has been performed by Mills and coworkers at Bell Laboratories (Chu, Mills and Hall, 1984 Fee et al, 1993a, b) following the first excitation of this transition by Chu and Mills (1982). Apart from various technicalities, the main difference between the 1984 and 1993 measurements was that in the latter a pulse created from a tuned 486 nm continuous-wave laser with a Fabry-Perot power build-up cavity, was used to excite the transition by two-photon Doppler-free absorption, followed by photoionization from the 2S level using an intense pulsed YAG laser doubled to 532 nm. Chu, Mills and Hall (1984), however, employed an intense pulsed 486 nm laser to photoionize the positronium directly by three-photon absorption from the ground state in tuning through the resonance. For reasons outlined by Fee et al. (1993b), it was hoped that the use of a continuous-wave laser to excite the transition would lead to a more accurate determination of the frequency interval than the value 1233 607 218.9 10.7 MHz obtained in the pulsed 486 nm laser experiment (after correction by Danzmann, Fee and Chu, 1989, and adjustment consequent on a recalibration of the Te2 reference line by McIntyre and Hansch, 1986). 

Several articles and reviews on different aspects of multi-photon excitation of biomolecule system are available. For example, Birch [11] consideraticms concentrate mainly on the impact of multi-photon techniques to the time-resolved fluorescence spectroscopy. Lakowicz and Gryczynski [12] have discussed examples of three-photon excited fluorescence. Rehms and Callis studied the two-photon excited fluorescence emission of aromatic amino acids [13]. Kierdasz et al analyzed emission spectra of Tyrosine- and Tryptophan-containing proteins using one-photon (270-3 10 nm) and two-photon (565-6 10 nm) excitation [14]. 

Fig. 8.16. Experimental spectrum of doubly-excited resonances in the barium spectrum obtained by three-photon spectroscopy. The horizontal arrows in the figure indicate lines which are not spectral features but frequency markers. Because of the mode of excitation, the lines tend to be more symmetrical than in some of the other spectra, but nevertheless exhibit a clear q reversal as the main feature is traversed. A theoretical fit by MQDT is also shown (after F. Gounand et al. [421]).

The development of multiphoton spectroscopy has followed that of lasers as the available power has increased, so has the number of photons involved in individual transitions. More significantly, it has become apparent that the physics of the interaction between radiation and matter is not the same at high laser powers as under weak illumination, i.e. that there is a qualitative change which sets in at strong laser fields. This is normally expressed by saying that perturbative approximations break down. A more direct (and equally accurate) statement is that new effects are observed, which are not present in conventional spectroscopy, even where the latter is extended to include, say, two- and three-photon transitions. 

CAHRS and CSHRS) [145. 146 and 147]. These 6WM spectroscopies depend on (Im for HRS) and obey the three-photon selection rules. Their signals are always to the blue of the incident beam(s), thus avoiding fluorescence problems. The selection rules allow one to probe, with optical frequencies, the usual IR spectrum (one photon), not the conventional Raman active vibrations (two photon), but also new vibrations that are symmetry forbidden in both IR and conventional Raman methods. 

Earlier absorption studies had identified a progression of parallel bands (traditionally labeled the C—X system) [181] in much the same energy region as the B—transition. These were long presumed to be associated with the 3pj(a 2) - 3aj promotion, but this presumption was shown to be invalid in 1978 following the discovery and characterization of the C—X system by Nieman and Colson [220]. The one-photon absorption spectrum shows no evidence of the C —X transition, but the spectroscopy of the first few i 2 levels of the C state of both NHj and ND3 is now well established from analysis of one-color two- and/or three-photon excitation spectra [227,228]. 

Surface Enhanced Hyper-Raman Spectroscopy (SEHRS) Hyper-Raman scattering is a nonlinear three-photon energy conversion process, that offers complementary informahon to Raman spectroscopy and has some advantages... 

Fig. 1.36 Level schemes of ionization spectroscopy (a) photoionization (b) excitation of autoion-izing Rydberg levels (c) two-photon ionization of excited molecules (d) one-photon ionization of a high lying level, excited by non-resonant two-photon process (e) three-photon excitation of a level which is ionized by a fourth photon (f) non-resonant two-photon ionization...

The three-photon absorption can be used for the excitation of high-lying molecular levels with the same parity as accessible to one-photon transitions. However, for a one-photon absorption, lasers with a wavelength A/3 have to be available in order to reach the same excitation energy. An example of Doppler-limited collinear three-photon spectroscopy is the excitation of high-lying levels of xenon and CO with a narrow-band pulsed dye laser at X = 440 nm (Fig. 2.40). For one-photon transitions light sources at A. = 146.7 nm in the VUV would have been necessary. 

A possible experimental arrangement for Doppler-free three-photon absorption spectroscopy is depicted in Fig. 2.41. The three laser beams generated by beam splitting of a single dye laser beam cross each other under 120° in the absorbing sample. 

Fig. 2.41 Possible arrangements for Doppler-free three-photon spectroscopy...

Three Photon Spectroscopy of the Lowest I dberg- and Valence States... 

THREE PHOTON SPECTROSCOPY OF THE LOWEST RYDBERG- AND VALENCE STATES OF THE CHLORINE MOLECULE... 

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