Resonance-enhanced multiphoton ionization REMPI of molecules

2 Resonance-enhanced multiphoton ionization (REMPI) of molecules 

It is quite natural to carry out the resonance photoionization of molecules via their electronically excited states. But this is not the only possibility. Polyatomic molecules can also be excited via their highly excited vibrational states (Bagratashvili et al. 1983). The former approach has found the widest application (Lubman 1990), thanks to the availability of high-power U V lasers, whereas the latter is not so widespread, as powerful tunable IR lasers are scarce. However, both these methods deserve a brief consideration. 

Successful experiments on the resonance-enhanced multiphoton ionization of polyatomic molecules by intense UV laser pulses with a tunable frequency were performed by Zandee and Bernstein (1979). Such experiments commonly use a time-of-flight 

19806). Aside from being highly efficient, the photoionization of benzene by excimer laser radiation is selective. For example, even at the maximum radiation intensities, such atmospheric components as H2 and H2O are practically completely absent in the mass spectra of the photoions. 

The mechanism of the multiphoton photoionization and fragmentation of polyatomic molecules by intense ( 10 W/cm ) UV laser pulses was studied in detail by Boesl et al. (1980). The spectroscopic features of resonance-enhanced multiphoton excitation and ionization were reviewed by Ashfold and Howe (1994). 

---