225.8 225.9 226.0 226.1 226.2 226.3 226.4 226.5 Wavelength (nm)

FIGURE 11.19 Two-color (1 + 1') REMPI spectrum of the NO (X2n, /■' = 0 -> A22 + , r' = 0) band at 300 K (adapted from Pfab, 1995).

ions. The Rydberg region can be reached by the simultaneous absorption of two or three photons, but with a loss of sensitivity (the absorption cross sections for two-photon absorption are of the order of 10

-50 cm4

compared to 10

cm for one-photon absorption).

On the other hand, pulse energies from conventional excimer or Nd:YAG pumped tunable laser systems are one to two orders of magnitude higher, since no second harmonic generation stages are required.

Other interferences from the use of two- or three-photon resonant excitation compared to (1 + n) processes are photolysis of the analyte at the energy of the first (or second) photon and broadening of the absorption spectra due to the higher photon fluxes employed.

Table 11.5 summarizes some potential REMPI processes for the measurement of species of atmospheric interest (Pfab, 1995). This ionization technique clearly has a great deal of potential, although to date it has not been applied extensively to measurements in ambient air.

c. Mass Filters

Two types of mass analyzers have been used extensively in atmospheric applications: quadrupole mass filters and time-of-flight (TOF) instruments. The use of ion traps is also being increasingly explored for this application. For the fundamental principles of mass

TABLE 11.5 Some Potential REMPI Transitions for Ionization of Molecules of Atmospheric Interest"

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