Resonant-Two-Photon-Ionization

James A M, Kowaiczyk P, Langlois E, Campbell M D, Ogawa A and Simard B 1994 Resonant two photon ionization spectroscopy of the molecules Nb, and Nb2 J. Chem. Rhys. 101 4485... 

Starting from 27, cyclo-Cig was prepared in the gas phase by laser flash heating and the neutral product, formed by stepwise elimination of three anthracene molecules in retro-Diels-Alder reactions, was detected by resonant two-photon-ionization time-of-flight mass spectrometry [23]. However, all attempts to prepare macroscopic quantities of the cyclocarbon by flash vacuum pyrolysis using solvent-assisted sublimation [50] only afforded anthracene and polymeric material. 

Kim NJ, Jeong G, Kim YS, Sung J, Kim SK, Park YD (2000) Resonant two-photon ionization and laser induced fluorescence spectroscopy of jet-cooled adenine. J Chem Phys 113 10051... 

Mons M, Piuzzi F, Dimicoli I, Gorb L, Lesczynski J (2006) Near-UV resonant two-photon ionization spectroscopy of gas phase guanine evidence for the observation of three rare tautomers. J Phys Chem A 110 10921-10924... 

In this chapter, we first present a brief overview of the experimental techniques that we and others have used to study torsional motion in S, and D0 (Section II). These are resonant two-photon ionization (R2PI) for S,-S0 spectroscopy and pulsed-field ionization (commonly known as ZEKE-PFI) for D0-S, spectroscopy. In Section HI, we summarize what is known about sixfold methyl rotor barriers in S0, S, and D0, including a brief description of how the absolute conformational preference can be inferred from spectral intensities. Section IV describes the threefold example of o-cholorotoluene in some detail and summarizes what is known about threefold barriers more generally. The sequence of molecules o-fluorotoluene, o-chlorotoluene, and 2-fluoro-6-chlorotoluene shows the effects of ort/io-fluoro and ortho-chloro substituents on the rotor potential. These are approximately additive in S0, S, and D0. Finally, in Section V, we present our ideas about the underlying causes of these diverse barrier heights and conformational preferences, based on analysis of the optimized geometries and electronic wavefunctions from ab initio calculations. 

Figure 1. Schematic of a two-color resonant two-photon ionization experiment "ZEKE states" lie just below each cation threshold, as indicated by asterisks.

The simplest version of REMPI uses a two photon (1-1-1) process, namely resonant two photon ionization (R2PI). In this, the species M is first promoted from its electronic ground state So to the electronic excited state Si via a resonant absorption step. Then, the non-resonant absorption of a second photon takes the species into the ionization continuum. If the frequencies of the excitation and ionization photons are equal, the process is named one color R2PI (lcR2PI), otherwise two colors R2PI (2cR2PI) (Fig. 1). 

One-color resonant two photon ionization (lcR2PI) spectroscopy... 

Tembreull R, Lubman DM. 1984. Use of resonant two-photon ionization with supersonic beam mass spectrometry in the discrimination of cresol isomers. Anal Chem 56(11) 1962-1967. 

Atmospheric pressure laser ionization (APLl) is a complementary technique to existing API methods. It is based on resonant or near-resonant two-photon ionization of aromatic ring systems. APLI utilizes resonantly enhanced multiphotonionization (REMPI) as the primary ion-production mechanism, nevertheless it occurs at atmospheric pressure [80],... 

Figure 2.5 Schematics of the concepts for (a) ionization-loss stimulated Raman spectroscopy (ILSRS), where resonant two photon ionization of the molecular parent, follows the depletion of ground state reactant species as a result of SRS and (b) VMP, where preexcited molecules are dissociated and the ensuing H photofragments are probed by (2 -I-1) resonantly enhanced multiphoton ionization. Reproduced with permission from Ref. [86]. Copyright (2009) AIP Publishing LLC.

In 1990, Zhu et al. [112] performed resonant two-photon ionization (R2PI) at 266 and 213 nm of substituted nitrobenzenes (NBs). Typically, in REMPI, the molecular ion itself is not observed for large parent molecules. They found that although extensive fragmentation occurs under vacuum, R2PI under atmospheric pressure (1 atm, He) has... 

Hansen, J.C. and Berry, R.S. (1984). Angular distribution of electrons from resonant two-photon ionization of molecules, J. Chem. Phys., 80, 4078-4096. 

Figure 12-2 shows data we obtained for three isolated GC base pair structures. Row A shows results for the Watson-Crick (WC) structure, while rows B and C represent the second and third lowest energy structures, respectively, which are not WC. The second column shows the IR-UV double resonance data, compared with the ab initio calculations of the vibrational frequencies. These data allow us to assign the structures. The third column shows the UV excitation spectra, measured by resonant two-photon ionization (R2PI). The UV spectrum is broad for the WC structure (A) and exhibits sharp vibronic lines for the other structures. 

---