Alexandra A. Hoops

Two- and three-body photodissociation of gas phase I3-

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.

Photodissociation and photoisomerization pathways of the HNCN free radical

The photodissociation spectroscopy and dynamics of the HNCN free radical have been investigated by fast beam photofragment translational spectroscopy. Predissociative transitions for both the B 2A′←X 2A″ band and a higher-energy band system assigned to the C 2A″←X 2A″ band were observed. Photofragment mass distributions indicate that N2 loss is the primary dissociation pathway. Translational energy distributions reveal a resolved vibrational structure of the N2 fragment, suggesting that the HNCN radical first isomerizes to a cyclic HCN2 intermediate. A dissociation mechanism is proposed in which electronically excited HNCN undergoes internal conversion to the ground state, followed by isomerization to cyclic HCN2 and dissociation through a tight three-center transition state. The HNCN bond dissociation energy D0 and heat of formation ΔfH0(HNCN) were determined to be 2.80±0.03 eV and 3.35±0.03 eV, respectively.

Photodissociation dynamics of the triiodide anion (I3

The spectroscopy and dissociation dynamics of I3− were investigated using fast beam photofragment translational spectroscopy. The photofragment yield of I3− from 420 to 240 nm was measured, yielding two broadbands at the same energies as in the absorption spectrum of I3− 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(2P3/2)+I2− and I*(2P1/2)+I2−. The 1:1 mass channel is from symmetric three-body dissociation to I−+2I.

Photodissociation dynamics of the CNN free radical

The spectroscopy and photodissociation dynamics of the A 3Π and B 3Σ− states of the CNN radical have been investigated by fast beam photofragment translational spectroscopy. Vibronic transitions located more than 1000 cm−1 above the A 3Π←X 3Σ− origin were found to predissociate. Photofragment yield spectra for the B 3Σ−←X 3Σ− band between 40 800 and 45 460 cm−1 display resolved vibrational progressions with peak spacing of ≈1000 cm−1 corresponding to symmetric stretch 10n and combination band 10n301 progressions. Ground state products C(3P)+N2 were found to be the major photodissociation channel for both the A 3Π and B 3Σ− states. The translational energy distributions for the A 3Π state are bimodal with high and low translational energy components. The distributions for the B 3Σ− state reveal partially resolved vibrational structure for the N2 photofragment and indicate extensive vibrational and rotational excitation of this fragment. These results suggest that bent geometries are involved in the d...

State-resolved translation energy distributions for NCO photodissociation

The photodissociation dynamics of NCO have been examined using fast beam photofragment translational spectroscopy. Excitation of the 102, 301, and 102302 transitions of the B 2Π←X 2Π band produces N(4S)+CO photofragments exclusively, while excitation of the 103303 transition yields primarily N(2D)+CO photoproducts. The translational energy [P(ET)] distributions yield D0(N–CO)=2.34±0.03 eV, and ΔHf,00(NCO)=1.36±0.03 eV. The P(ET) distributions exhibit vibrationally resolved structure reflecting the vibrational and rotational distributions of the CO product. The N(2D)+CO distribution can be fit by phase space theory (PST), while the higher degree of CO rotational excitation for N(4S)+CO products implies that NCO passes through a bent geometry upon dissociation. The P(ET) distributions suggest that when the B 2Π←X 2Π band is excited, NCO undergoes internal conversion to its ground electronic state prior to dissociation. Excitation of NCO at 193 nm clearly leads to the production of N(2D)+CO fragments. Wh...

Photodissociation dynamics of the HCNN radical

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 DeltafH0(HCNN)=5.010+/-0.023 eV, were determined.

Excited states and photodissociation dynamics of the triiodine radical (I3)

The electronic spectroscopy and photodissociation dynamics of the I3 radical are investigated with two experimental methods. The ground and several low-lying excited states of the I3 radical are characterized by photoelectron spectroscopy of I3− at 213 nm. Assignments of these states are discussed with reference to recent calculations. In addition, photodissociation of the I3 radical was investigated at selected photon energies (4.59, 4.96, and 5.17 eV) by fast radical beam photofragment translational spectroscopy. Two product channels were observed with mass ratios of 1:2 and 1:1, and translational energy (P(ET)) distributions were measured. The P(ET) distributions for products with mass ratio 1:2 show that this channel corresponds to I2 in various electronic states along with atomic I in its 2P3/2 or 2P1/2 state. The 1:1 channel corresponds primarily to concerted three-body dissociation to three I atoms.

Detection of mercuric chloride by photofragment emission using a frequency-converted fiber amplifier

A real-time, noninvasive approach for detecting trace amounts of vapor-phase mercuric chloride (HgCl(2)) in combustion flue gas is demonstrated using a near-infrared pulsed fiber amplifier that is frequency converted to the ultraviolet. Excitation of the HgCl(2) ([see text]) transition at 213 nm generates 253.7 nm emission from the Hg (6(3)P(1)) photoproduct that is proportional to the concentration of HgCl(2). A measured quadratic dependence of the HgCl(2) photofragment emission (PFE) signal on the laser irradiance indicates that the photodissociation process involves two-photon excitation. Additionally, low concentrations of HgCl(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.

Development of a compact narrow-linewidth tunable ultraviolet laser source for detection of Hg0.

A portable laser for real-time, stand-off detection of Hg0emissions from coal-fired power plants is developed and characterized. The pulse energy of the 254-nm laser is 1.8 μJ, which will enable sub-ppb detection of Hg0.