Resonant two-photon ionization R2PI

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. 

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). 

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... 

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. 

Experiments with clusters in a supersonic jet can advantageously be employed to study the effects of solvation of a chromophore on its emission spectrum. Hence it is desirable to characterize the composition and the structure of the solute-solvent clusters as precisely as possible. The cluster size distributions can be determined by TOF-MS after resonant two-photon ionization, R2PI [84, 92a,c]. This allows for a... 

Resonant two-photon ionization (R2PI) The skimmed core of the expansion was passed into a second differentially pumped vacuum chamber (p<5 10" ), which housed a 44 cm long space-focusing time of flight mass spectrometer. 

Cluster synthesis by supersonic expansion normally yields broad distribution of cluster sizes. The measurement of any size-dependent property depends on the availability of a mass-specific detection scheme [20]. Relatively nondestructive and selective methods for assaying neutral cluster size distributions have been provided by combinations of mass spectrometry with (a) laser techniques (resonant two-photon ionization (R2PI) [21-26], depletion spectroscopy [27]) and (b) helium cross-beam scattering techniques [28]. 

FIGURE 6.4 Resonant two-photon ionization (R2PI) spectra of (a) indole-3-acetic acid and (b) 5-hydroxyindole-3-acetic acid. (Reprinted with permission from reference 14). 

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