Laser induced fluorescence from NH2(2A1). State selected radiative lifetimes and collisional de-excitation rates

Abstract

Time resolved fluorescence from the first excited (2A1) state of NH2 has been observed following excitation of the radical in its ground state by means of a pulsed tunable dye laser. Specific rotational levels within a number of vibronic states were populated, decay rates measured as a function of total pressure for a variety of added gases, and zero pressure lifetimes and collisional de-excitation rates evaluated. Measured zero pressure lifetimes are good approximations to the vibrational state radiative lifetimes, typically 10 μsec for the (0, 9, 0) state. Collisional de-excitation rate constants were measured as 1.0×10−9 cm3 molecule−1⋅sec−1 for NH3, independent of vibronic state, and for the Σ (0, 9, 0) level were found for other gases in the ratio NH3:CO:H2:N2:CH4:Ar:He=1.0:0.47:0.46:0. 40:0.30:0.152:0.145. Using excitation by a tunable cw dye laser, steady state spectra of NH2 have been obtained and collisional energy transfer observed within the (2A1) excited electronic state of NH2. Transfer was observed both within the initially populated vibronic state and to other such states within the same overall vibrational level. The symmetric or antisymmetric character of the rotational level remained unchanged in collision, i.e., only a↔a and s↔s transfer occurred.

Optical emission in the dissociation of ammonia by low energy He+ ions

“…it is about 10 μs [54]. In our arrangement, a lifetime of this order allows for a massive (>98%) escape of emitters from the observation window.…”

Non-stationary states in molecular spectroscopy: The Hanle effect in NO2

“…Hanle experiments in NH 2 revealed a lifetime zc of about 0.35 gs in several rotational levels of the (0, 10,0) vibrational level of the 2A 1 electronic state, when the g factor was evaluated assuming Hund's coupling case b [4]. Lifetime measurements by time resolved fluorescence in the (0, 9,0) vibrational level of the same electronic state gave r R values of about 10gs [5]. Lifetime measurements in CS 2 [6][7][8][9] with both methods also seem to show that rc is about a factor 3 smaller than ~c R. This conclusion is suggested by the fact that this assumption removes the discrepancy between the rate for collisional quenching of fluorescence and the rate for destruction of coherence by collisions.…”

Reactivity and kinetic properties of the NH2 radical in the gas phase

“…B. Laser induced fluorescence from the excited NH~(2A1)_ state has been observedunder variThe emission ous conditions [56] and particularly in the vacut~Wphotolysis of an~nonia (a bands) [7]. By tuning a dye laser on a strong absorption line of NH2X(2B1) emission from the A(2A1) excited state is obtained [42,57,58]. This an intense laser induced fluorescence is to date the most sensitive method for detecting h~t 2 radicals at low pressure (detection limit < 109 molecules cm-3).…”

“…The radiative lifetime of the A(2A1) state has been measured under various conditions: direct photolysis of an~onia [59][60][61] and laser induced fluorescence in bulk [58] or in a supersonic free jet [62]. It varies from roughly 5 to 30 ~s according to the emitting vibrational or rotational levels.…”

“…It varies from roughly 5 to 30 ~s according to the emitting vibrational or rotational levels. This lifetime is fairly long so that the A(2AI) excited state can undergo efficient collisional deactivation since the quenching rate approaches the collision frequency [58,61]. As a result, the detection of ~2 by laser induced fluorescence is limited to low pressures (< 10-100 Torr).…”