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The effects of collisional quenching on photofragment emission (PFE) detection of vapor-phase Hg[Cl.sub.2] in combustion flue gas constituents are investigated. Exciting Hg[Cl.sub.2] via the 1 [sup.1][[PI].sub.u] [left arrow] 1 [sup.1][[summation].sup.+.sub.g] transition, time-resolved measurements of emission from the Hg([6.sup.3][P.sub.1]) daughter in buffer-gas mixtures of [N.sub.2], [O.sub.2], and C[O.sub.2] indicate that the fragmentation pathway passes through a long-lived intermediate species, which we assign to Hg([6.sup.3][P.sub.2]). Total quenching rate coefficients of Hg([6.sup.3][P.sub.1]) by [N.sub.2], [O.sub.2], and C[O.sub.2] are consistent with values reported in the literature. In addition, total quenching rate coefficients for the intermediate Hg([6.sup.3][P.sub.2]) state are determined to be 1.72([+ or -] 0.08) x [10.sup.-10] [cm.sup.3] [molecule.sup.-1] [s.sup.-1] and 2.90([+ or -] 0.37) x [10.sup.-10] [cm.sup.3] [molecule.sup.-1] [s.sup.-1] for [N.sub.2] and [O.sub.2], respectively. An analysis of the impact of the collisionally dependent energy-transfer process that precedes the formation of Hg([6.sup.3][P.sub.1]) on the use of PFE to measure Hg[Cl.sub.2] concentration is presented. OCIS codes: 280.1120, 300.2530, 120.0280.

A real-time, noninvasive approach for detecting trace amounts of vapor-phase mercuric chloride (Hg[Cl.sub.2]) in combustion flue gas is demonstrated using a near-infrared pulsed fiber amplifier that is frequency converted to the ultraviolet. Excitation of the Hg[Cl.sub.2] ([1.sup.1][[PHI].sub.[micro]] [left arrow] [1.sup.1] [[SIGMA].sup.+.sub.g]]) transition at 213 nm generates 253.7 nm emission from the Hg ([6.sup.3][P.sub.1]) photoproduct that is proportional to the concentration of Hg[Cl.sub.2]. A measured quadratic dependence of the Hg[Cl.sub.2] photofragment emission (PFE) signal on the laser irradiance indicates that the photodissociation process involves two-photon excitation. Additionally, low concentrations of Hg[Cl.sub.2] are detected with the PFE approach in an environment characteristic of coal-fired power-plant flue gas using this compact solid-state laser source. A detection limit of 0.7 ppb is extrapolated from these results. OCIS codes: 280.1120, 300.2530, 120.0280.

The viability of pulsed laser photofragment emission (PFE) is evaluated for the in situ measurement of vapor-phase mercuric chloride (Hg[Cl.sub.2]) concentration in combustion flue gas. Dispersed emissions from both the Hg ([6.sup.3][P.sub.1]) and HgCl ([B.sup.2] [[SIGMA].sup.+] photoproducts are presented, and the dependence of the Hg[Cl.sub.2] PFE signal originating from Hg ([6.sup.3][P.sub.1]) on the collisional environment is examined for buffer-gas mixtures of [N.sub.2], [O.sub.2], and C[O.sub.2]. Integrated PFE intensity measurements as a function of buffer gas pressure support the assumption that the primary effect of the relevant flue gas constituents is to quench emission from Hg ([6.sup.3][P.sub.1]). The quenching rate constants for PFE from Hg[Cl.sub.2] were measured to be 1.37 ([+ or -] 0.16) x [10.sup.5] [Torr.sup.-1] [s.sup.-1] for [N.sub.2], 9.35 ([+ or -] 0.25) X [10.sup.6] [Torr.sup.-1] [s.sup.-1] for [O.sub.2], and 1.49 ([+ or -]) X [10.sup.6] [Torr.sup.-1] for C[O.sub.2]. These values are in good accord with literature values for the quenching of Hg ([6.sup.3][P.sub.1]). The emission cross section for Hg ([6.sup.3][P.sub.1]) generated by photodissociation of Hg[Cl.sub.2] in 760 Torr [N.sub.2] is found to be 1.0 ([+ or -] 0.2) X [10.sup.-25] [m.sup.2] by comparing the PFE signal to [N.sub.2] Raman scattering. OCIS codes: 280.1120, 300.2530, 120.0280.

Issues concerning the photodissociation of mass-selected neutral carbon clusters C4, C5 and C6 are discussed. It is established that multiphoton absorption arises over a wide range of photon energies.