SPECTROCHIMICA ACTA PART B ELSEVIER Spectrochimica Acta Part B 52 (1997) 295-304 Analytical time-resolved laser enhanced ionization spectroscopy I. Collisional ionization and photoionization of the Hg Rydberg states in a low pressure gas W.L. Clevenger, L.S. Mordoh, O.I. Matveev a, N. Omenetto 2, B.W. Smith, J.D. Winefordner* Department of Chemistry, University of Florida, Gainesville, FL 32611-7200, USA Received 7 June 1996; accepted 4 September 1996 Abstract The temporal behavior of the laser enhanced ionization signal of mercury was studied in a quartz cell under low buffer gas pressure. Using fast electronics and a short (34 ns) laser pulse, it was possible to distinguish, in one single time-resolved ionization waveform, the non-selective photoionization component of the signal from that which was due to collisional ionization from selected levels. Experimental results were shown to agree with those obtained by computer simulation, and optimal conditions for deconvolution of the two components were studied. © 1997 Elsevier Science B.V. Keywords: Mercury; Time-resolved Ionization; Rydberg Levels; Photoionization; Collisional Ionization I. Introduction In laser enhanced ionization (LEI) spectroscopy [1], the process of ion formation due to collisional ionization of analyte atoms can practically always be accompanied by their resonance photoionization and by non-selective ionization of molecules and/or atoms of buffer gas. Often, in order to improve the limit of detection, one needs to acquire separate infor- mation about the contribution of the LEI signal bearing useful analytical information and the signal due to non-selective photoionization. This subject has been treated in several papers, together with the * Corresponding author. ~On leave from Department of Chemistry, Moscow State University, Moscow, Russia. 2 Present address: Environment Institute, Joint Research Centre, Ispra, (Va) Italy. different concepts associated with the two definitions of ion yield and ionization efficiency [2-6]. Most work has been performed in flames operated at atmospheric pressure. Recently, it has been suggested [7] that when using flames as atomizers, this informa- tion can be obtained by observing a single time- resolved ionization waveform under conditions when the faster photoionization component and the slower collisional ionization component of the signal can be distinguished from one another. This idea can be extended to other atom reservoirs, such as a low pressure buffer gas. The deconvolution becomes pos- sible when (1) the duration of the laser pulse is shorter than the average time in the collisional ionization process and (2) the distance between the collection electrode and the laser beam is less than the average distance traveled by the electron as a result of its drift velocity during the process of the collisional 0584-8547/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved, PH $0584-8547(96)01589-3