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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.

The photodissociation dynamics of the diazomethyl (HCNN) radical have been studied using fast radical beam photofragment translational spectroscopy. A photofragment yield spectrum was obtained for the range of 25 510–40 820 cm-1, and photodissociation was shown to occur for energies above 25 600 cm-1. The only product channel observed was the formation of CH and N2. Fragment translational energy and angular distributions were obtained at several energies in the range covered by the photofragment yield spectrum. The fragment translational energy distributions showed at least two distinct features at energies up to 4.59 eV, and were not well fit by phase space theory at any of the excitation energies studied. A revised C–N bond dissociation energy and heat of formation for HCNN, D0(HC–NN)=1.139±0.019 eV and ΔfH0(HCNN)=5.010±0.023 eV, were determined. [ABSTRACT FROM AUTHOR]

The photodissociation spectroscopy and dynamics resulting from excitation of the B 2A″←X 2A″ transition of CH2CFO have been examined using fast beam photofragment translational spectroscopy. The photofragment yield spectrum reveals vibrationally resolved structure between 29 870 and 38 800 cm-1, extending ∼6000 cm-1 higher in energy than previously reported in a laser-induced fluorescence excitation spectrum. At all photon energies investigated, only the CH2F+CO and HCCO+HF fragment channels are observed. Both product channels yield photofragment translational energy distributions that are characteristic of a decay mechanism with a barrier to dissociation. Using the barrier impulsive model, it is shown that fragmentation to CH2F+CO products occurs on the ground state potential energy surface with the isomerization barrier between CH2CFO and CH2FCO governing the observed translational energy distributions. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of I3- from 390 to 290 nm (3.18 to 4.28 eV) have been investigated using fast beam photofragment translational spectroscopy in which the products are detected and analyzed with coincidence imaging. At photon energies ≤3.87 eV, two-body dissociation that generates I-+I2(A 3Π1u) and vibrationally excited I2-(X 2Σu+)+I(2P3/2) is observed, while at energies >=3.87 eV, I*(2P1/2)+I2-(X 2Σu+) is the primary two-body dissociation channel. In addition, three-body dissociation yielding I-+2I(2P3/2) photofragments is seen throughout the energy range probed; this is the dominant channel at all but the lowest photon energy. Analysis of the three-body dissociation events indicates that this channel results primarily from a synchronous concerted decay mechanism. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of I3- from 390 to 290 nm (3.18 to 4.28 eV) have been investigated using fast beam photofragment translational spectroscopy in which the products are detected and analyzed with coincidence imaging. At photon energies ≤3.87 eV, two-body dissociation that generates I-+I2(A 3Π1u) and vibrationally excited I2-(X 2Σu+)+I(2P3/2) is observed, while at energies >=3.87 eV, I*(2P1/2)+I2-(X 2Σu+) is the primary two-body dissociation channel. In addition, three-body dissociation yielding I-+2I(2P3/2) photofragments is seen throughout the energy range probed; this is the dominant channel at all but the lowest photon energy. Analysis of the three-body dissociation events indicates that this channel results primarily from a synchronous concerted decay mechanism. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The spectroscopy and dissociation dynamics of I[sub 3][sup -] were investigated using fast beam photofragment translational spectroscopy. The photofragment yield of I[sub 3][sup -] from 420 to 240 nm was measured, yielding two broadbands at the same energies as in the absorption spectrum of I[sub 3][sup -] in solution. Photodissociation dynamics measurements performed with two-particle time-and-position sensitive detection revealed two product mass channels having photofragment mass ratios of 1:2 and 1:1. Both channels were seen at all photolysis wavelengths. Translational energy distributions show that the 1:2 products are from a combination of I([sup 2]P[sub 3/2])+I[sub 2][sup -] and I[sup *]([sup 2]P[sub 1/2])+I[sub 2][sup -]. The 1:1 mass channel is from symmetric three-body dissociation to I[sup -]+2I. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of bare I2− and I2− · Ar at 413 and 390 nm have been investigated using a fast beam instrument coupled with a new photofragment coincidence imaging detector. Results from the application of this technique to the dissociation of I2− and I2− · Ar yielded the dissociation energy of I2− (D0(I2−)=1.012±0.008 eV) and I2−–Ar binding energy (D0(I2−–Ar)=45±8 meV). The experiments show that at these wavelengths, I2− · Ar undergoes three-body dissociation to I− + I* + Ar, with very low momentum in the Ar atom and unequal momentum partitioning between the two I atoms. [Copyright &y& Elsevier]