Resonantly enhanced two-photon

Weber, Th., von Bargen, A., Riedle, E., and Neusser, H. J. (1990), Rotationally Resolved Ultraviolet Spectrum of the benzen-Ar Complex by Mass-Selected Resonance-Enhanced Two-Photon Ionization, J. Chem. Phys. 92,90. 

Figure 4-10. Resonance enhanced two-photon ionization spectra of l-naphthol(NH3) clusters (n = 1-3). l-naphthol(NH3)2 shows two 0° origins corrseponding to two different isomers (from Cheshnovsky and Leutwyler 1988).

Figure 4-19. Resonance-enhanced two-photon ionization spectra of ions issued from fluorobenzene/methanol d4/helium clusters, measured by scanning the laser near the 00 transition of fluorobenzene [0]. Bands [4-6] are due to the FB+(CD3OD)2 precursor which totally fragments, either by evaporation of one methanol-d4 molecule or reaction leading to anisole + DF + CD3OD. Bands [7-9] are due to the 1-3 precursor also losing one CD3OD molecule or reacting. Bands [1-3] are more likely attributed to the 1-1 complex (isomer( ) [1], hot band [2], 0q [3]) (from Brutschy et al. 1991).

Leahy, D.J., Reid, K.L. and Zare, R.N. (1991). Effect of breaking cylindrical symmetry on photoelectron angular distribution resulting from resonance-enhanced two-photon ionization, J. Chem. Phys., 95, 1746-1756.,... 

TheM vs. N photon scenarios, where both routes are nonresonant, were discussed in Section 3.3. Here we consider resonantly enhanced routes and show, in particular,, jhat a resonantly enhanced two-photon vs. two-photon excitation (see Fig. 6.1) provides a means of maintaining control in a molecular system in thermal equilibrium, The resonant character of the excitations ensure that only a particular initial... 

Figure 6.1 Resonantly enhanced two-photon vs. two-photon control scenarios (a) using J four frequencies and (h) using three frequencies. /...

Resonantly enhanced two-photon dissociation of Na2 from a bound state of the. ground electronic state occurs [202] by initial excitation to an excited intermediate bound state Em,Jm, Mm). The latter is a superposition of states of the A1 1+ and b3Il electronic curves, a consequence of spin-orbit coupling. The continuum states reached in the two-photon excitation can have either a singlet or a triplet character, but, despite the multitude of electronic states involved in the computation reported J below, the predominant contributions to the products Na(3s) + Na(3p) and Na(3s) + Na(4s) are found to come from the 1 flg and 3 + electronic states, respectively. The resonant character of the two-photon excitation allows tire selection of a Single initial state from a thermal ensemble here results for vt = Ji — 0, where vt,./, denote the vibrational and rotational quantum numbers of the initial state, are stJjseussed. 

The significance of resonantly enhanced two-photon association stems from the visibility of using it to form ultracold molecules, a topic of considerable interest. fSr cooling schemes that work for atoms [338-340] tend to fail for molecules, ily due to the presence of many near-resonance lines and the presence of other ges, of freedom, in addition to translation (rotations, vibrations, etc.), that must... 

Fig. 5. Resonance enhanced, two-photon spectrum of rotationally relaxed NO produced in reaction (18) initiated by the KrF laser...

Figure 7. Schematic energy level diagram showing the principle of the ionization method for detecting electron transfer in gas-phase adducts. Naphthalene cation (the hole donor) is formed by resonance-enhanced two-photon ionization of the neutral. A hole acceptor, whose ionization potential is lower than that of naphthalene, is not ionized, since its S level is not resonant with the UV photons used (vi). The vibrational levels of the ionic form of the acceptor are resonant with the naphthalene cation, and accept the hole easily. Detection is by photodissociation, using photons of different frequency (V2) that dissociate the naphthalene cation in a resonance-enhanced multiphoton absorption process. Charge transfer is detected by the diminution of the product ion signal in the presence of a suitable acceptor. Adapted from Ref. [32].

Fig. 9.1. Resonantly enhanced two-photon ionisation in Na. Two lasers are used the sum of the two frequencies is fixed, and the intermediate energy is tuned through the 3p doublet. The inset shows the region close to resonance (after J.E. Bjorkholm and P.F. Liao [466]).

Multiphoton ionization spectroscopy has been reviewed in two recent articles. An apparatus has been described for constant intensity multiphoton ionization spectroscopy. A sensitive molecular vapour detection system utilizing resonance-enhanced two-photon ionization has been used to monitor naphthalene to a limit of 5 x 10 molecules cm Excitation was achieved... 

Lubman, D.M. Kronick, M.N., Resonance-enhanced two-photon ionization spectroscopy in plasma chromatography, AmZ. Chem. 1983, 55, 1486-1492. 

Anex, D.S., de Vries, M.S., KnebeUcamp, A., Bargon, J., Wendt, H.R., and Hunziker, H.E., "Resonance-Enhanced Two-Photon Ionization Time-of-Flight Spectroscopy of Cold Perfluorinated Polyethers and Their External and Internal Van der Waals Dimers," International Journal of Mass Spectrometry and Ion Processes. 131,319-334, 1994. 

Kjcergaard, N. Homekaer L. Thommesen, A.M. Videsen, Z. Drewsen, M. Isotope selective loading of an ion trap using resonance-enhanced two-photon ionization. Appl. Phys. B. 2000, 71, 207-210. 

R2PI resonance-enhanced two-photon ionization spectroscopy... 

Massick and Breckenridge [97Mas] have carried out Resonance Enhanced Two-Photon (R2PI) spectroscopy starting from the two metastable states Mg(3s3pjt Ar( IIo+, IIo-). They ended in four Rydberg states ... 

Figure 1 Photoelectron spectra of benzene isotopes C5H5, C5D5H and 6 6 after resonance-enhanced two-photon ionization...

Benzene ions are produced in an effusive molecular beam inside the acceleration field of a reflectron time-of-flight mass spectrometer /12/. (see Fig. 3). Laser 1 is tuned to the frequency of the 6 i or 60116 -1 band and produces, via a resonance-enhanced two photon absorption, state- and energy-selected benzene cations. 

Figure 2 Excitation scheme for the two laser pump-pump experiment. State-selected benzene cations are produced in a resonantly enhanced two-photon ionization process. A second laser pulse of variable photon energy excites the ions to a well defined energy level above the dissociation threshold and metastable dissociation takes place (taken from ref. /16/).

In conclusion, we have shown that resonantly enhanced two-photon ionization is a versatile method for the production of state- and energy-selected polyatomic molecular ions. This was explicitly demonstrated by an analysis of the kinetic energy distribution of the ejected photoelectrons. In a reflectron time-of-flight mass spectrometer the total decay rate constants and individual decay rate constants of internal energy-selected molecular ions have been measured for various well defined internal energies. From our experimental results detailed information about the statistical character of the dissociation mechanism and the structure of the activated complex is obtained. 

The results on crystal structures are not the only experimental evidences of the existence of X-H- - - k interactions. There are numerous gas-phase experimental studies. For example, the high-resolution optical and microwave spectra on the benzene-ammonia complex were presented [9]. It was found that in the vibrationally averaged structure, the C3 symmetry axis of ammonia is tilted by approximately 58° relative to the Ce benzene axis. In such a way the N-H bonds interact with rr-electrons of benzene through N-H- - - k hydrogen bonds. The resonance-enhanced two-photon ionization, the microwave spectroscopy and the other spectroscopic techniques were used to analyze such complexes as C6H6-H2O [10], CeHe-HF [11], CeHs-HCl [12] and also T-shaped complexes where rr-electrons of acetylene act as the proton acceptor while such moieties as HF, HCl or HCN are the proton donors [13-15]. 

Resonance enhanced two-photon ionization via the A <— X y(O-O) and v(l-l) bands was used for state-specific detection of NO. Frequency doubling the output of a XeCl excimer pumped dye laser in a potassium pentaborate crystal produced tunable ultraviolet radiation for the ionization with UV pulse energies of approximately 30 microjoules in a bandwidth of about 0.4 cm. The focussed UV beam crossed the molecular beam at right angles and could be moved over a variety of radii and angles about the scattering sample surface. Ionized NO molecules were detected using a Johnston MM-1 multiplier. 

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