J. Phys. Chem. 1996, 100, 8061-8063
8061
Direct Detection of Oxygen 1D2 Atoms by Mass-Resolved, Resonance-Enhanced Multiphoton Ionization Spectroscopy, following Ozone Photolysis at ∼276 nm R. C. Richter and A. J. Hynes* DiVision of Marine and Atmospheric Chemistry, Rosenstiel School of Marine and Atmospheric Science, UniVersity of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149 ReceiVed: March 13, 1996X
Singlet atomic oxygen Rydberg states of the (2D) 3d′ configuration have been observed as three-photon resonances. The states lie above the first ionization energy but are forbidden to autoionize in LS coupling. All transitions originated from the metastable p4 1D2 state produced by laser photolysis of ozone at ∼276 nm. The same laser pulse was used to ionize the atoms. Arrival time spectra of the atomic photofragment have also been recorded.
Introduction The importance of metastable atomic oxygen (1D2) in various atmospheric processes was recognized many years ago.1,2 Its reactions with several species have been measured, as have the physical quenching rates to produce O(3P).3 Most experiments were performed using indirect detection of O(1D) atoms, either by monitoring reaction products or following the time dependence of the O(3P) concentration.3,4 Direct detection has also been used (vacuum-UV absorption at ∼115.2 nm5 or emission at ∼630 nm6), but the applicability of these methods is limited by experimental difficulties and/or low sensitivity. More recently Pratt et al. have used multiphoton excitation of O(1D) in the 203-205 nm region to study singlet Rydberg states of atomic oxygen by multiphoton ionization7 and double-resonance spectroscopy.8 Low-lying singlet states of atomic oxygen arising from the ground-state p4 configuration (1D and 1S) cannot ionize directly to form the 4S3/2 ground state of O+.7-10 Members of Rydberg series converging to the next two higher states of the ion (2D and 2P) all (except the (2D) 3s′ state) lie above the first ionization potential but have been observed in both absorption and emission,11,12 indicating that their lifetimes are long. Ultraviolet ozone photolysis has been studied by many laboratories in recent years.13-17 While the absorption cross section is known with high accuracy over the entire wavelength range 170-360 nm,3,18 there still exists some controversy about the relative yields of the photoproducts in the long wavelength tail above 310 nm.3,16 The two spin-allowed processes are
O3 f O(1D) + O2(a1∆g)
(1)
O3 f O(3P) + O2(X3Σ-g)
(2)
the former being dominant at wavelengths below 310 nm. The average quantum yield for O(1D) production in the range 266311 nm has been reported as 0.89 ( 0.03.15 The currently recommended NASA3 value for the range 290-305 nm is 0.95. The molecular photofragment has been observed directly by various techniques (CARS,15 REMPI,16 TOF17), and its internal energy distribution has been reported. In this study we have used transitions to (2D) 3d′ states to selectively detect O(1D) * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, May 1, 1996.
S0022-3654(96)00756-3 CCC: $12.00
atoms produced by ozone photolysis, using multiphoton ionization spectroscopy. Experimental Section The experiments described here were performed in a conventional GC/MS (HP Model 5980A) with a (homemade) modified ion source (Figure 1). Quanta-Ray PDL-2 dye laser, pumped by the second harmonic output of a Quanta-Ray GCR16S Ng:YAG laser, was used as a light source. The output of the dye laser was frequency doubled using a KD*P crystal, and focused into the ion source of the mass spectrometer. The laser power density in the focus could be varied by using lenses with a focal length in the range 8-30 cm or changing the pump laser Q-switch delay. The highest UV laser energy was approximately 1 mJ/pulse, and the specified line width of the dye laser fundamental output was 0.25 cm-1. The polarization of the UV beam was rotated for some experiments using a λ/2 waveplate. Multiphoton ionization spectra were recorded by monitoring the O+ ion current while scanning the dye laser wavelength. The UV output of the laser was continuously monitored during the experiment and found to be constant over the small wavelength range used. The spectra were not corrected for pulse-to-pulse power fluctuations (
