Influence of the focal length of the laser beam focusing lens on MPI yield

Abstract

For a multiphoton ionization (MPI) process which follows an nth-order laser power law ( n ⩾ 2), the ionization yield at any given value of the laser power depends upon the focal length, f, of the the focusing lens. For a spherical lens it is shown that, for any fixed laser power, the MPI intensity is proportional to f 4−2 n . Thus it is possible to determine the power law index, n, from the slope, s, of a log-log plot of the ion signal versus f (at constant laser power) via the equation n = 2 − s/2. Confirmatory experiments have been carried out using a MPI time-of-flight mass spectrometer with two series of laser beam focusing lenses, with f in the range 17–50 cm. Results are presented for the MPI of triethylenediamine (DABCO) at 532 nm (a 2 + 2 REMPI process), for Hg at 553.88 nm (a 2 + 3 REMPI), Hg at 532 nm (non-resonant, n = 5), and Hg at 280.39 nm (a 2 + 1 REMPI). The f 4−2 n relationship is independent of the laser beam profile (uniform, gaussian or annular). The importance for MPI of coherence of the laser radiation is demonstrated.