Measurement of the photoionization spectra and ionization thresholds of the H sub 2 CN and D sub 2 CN radicals

The photoionization spectra of the H{sub 2}CN and D{sub 2}CN radicals were obtained by photoionization mass spectroscopy (PIMS) using synchrotron radiation. The radicals were generated by the reaction of N with CH{sub 3} and CD{sub 3}, respectively. For both H{sub 2}CN and D{sub 2}CN a prominent feature was observed near 118.6 nm (10.5 eV) and the ionization threshold was determined to be 9.4 {plus minus} 0.1 eV; both features provide additional signatures for identifying H{sub 2}CN in complex systems. By use of a corrected value for {Delta}H{sub f} (H{sub 2}CN) derived from a recent electron affinity measurement and other available measured or calculated thermochemical quantities for H{sub 2}CN and HCNH radicals and radical ions, a value of 10.8 {plus minus} 0.6 eV for the ionization energy of H{sub 2}CN was derived. The much lower value derived for the ionization energy of HCNH (6.8 and 7.0 eV for the cis and trans isomers, respectively) is consistent with the product of the N + CH{sub 3} reaction being the H{sub 2}CN isomer and not HCNH. The ionization threshold observed at 9.4 eV is attributed to autoionization arising from high Rydberg states of H{sub 2}CN which couple into vibrationally excited states of the linear HCNH{sup +} ground state of the ion. Also discussed are the roles of the H{sub 2}CN radical and HCNH{sup +} radical ion in the chemistry of the atmosphere of Titan and in interstellar clouds.