<title>New ways to optically detect CH2 and HCF radicals using resonance-enhanced multiphoton ionization spectroscopy</title>

Abstract

We report two new optical detection methods for the CH<SUB>2</SUB> (X <SUP>3</SUP>B<SUB>1</SUB>) radical and one new detection method for the HCF (X <SUP>1</SUP>A') radical. These detection methods are based upon resonance enhanced multiphoton ionization (REMPI) spectroscopy and require only commonly available lasers. Single laser pulse sensitivity for CH<SUB>2</SUB> and HCF is better than 10<SUP>8</SUP> radicals(DOT)cm<SUP>-3</SUP>. REMPI spectra of CH<SUB>2</SUB> between 380 and 440 nm arose from three-photon resonances with the B <SUP>3</SUP>A<SUB>2</SUB> (3d), C (3d), D (3d), and 4d <SUP>3</SUP>A<SUB>2</SUB> Rydberg states between 78,950 and 68,200 cm<SUP>-1</SUP> above the ground state. A fourth laser photon ionized the radicals and the signal was carried by CH<SUB>2</SUB><SUP>+</SUP> (m/z 14) ions, i.e., CH<SUB>2</SUB> signal arises through a 3 + 1 REMPI mechanism. We have also discovered two new states, H(3p) and I(4p), that enable extremely sensitive detection of CH<SUB>2</SUB> through a 2 + 1 REMPI scheme. In two photons, the H(3p) state produces a strong band at 311.80 nm (64,126 cm<SUP>-1</SUP>). The much weaker I(4p) state appears at 269.27 nm (74,254 cm<SUP>-1</SUP>). Between 305 and 325 nm HCF and DCF radicals produce 2 + 1 REMPI spectra by excitation to 3p Rydberg states. The band origins for HCF and DCF are at 321.6 nm (62,180 cm<SUP>-1</SUP>) and 321.7 nm, respectively.