Beam Photofragment Translational Spectroscopy Research Articles

Photodissociation of iso-propoxy (i-C3H7O) radical at 248 nm.

Photodissociation of the i-C3H7O radical is investigated using fast beam photofragment translational spectroscopy. Neutral i-C3H7O radicals are produced through the photodetachment of a fast beam of i-C3H7O- anions and are subsequently dissociated at 248 nm (5.0 eV). The dominant product channels are CH3 + CH3CHO and OH + C3H6 with some contribution from H + C3H6O. The CH3 and H loss channels are attributed to dissociation on the ground electronic state of i-C3H7O, but in a nonstatistical manner because calculated RRKM dissociation rates exceed the rate of energy randomization. Translational energy and angular distributions for OH loss are consistent with ground state dissociation, but the branching ratio for this channel is considerably higher than predicted from RRKM rate calculations. Additionally, i-C3H7O undergoes three-body fragmentation to CH3 + CH3 + HCO and CH3 + CH4 + CO. These three-body channels are attributed to dissociation of i-C3H7O to CH3 + CH3CHO, followed by secondary dissociation of CH3CHO on its ground electronic state.

Photodissociation dynamics of the methylsulfinyl radical at 248 nm

The photodissociation dynamics of jet-cooled methylsulfinyl radicals, CH3SO, at 248 nm have been investigated using molecular beam photofragment translational spectroscopy. The primary channel is CH3S + O, which occurs via the initially prepared excited CH3SO state by rapid cleavage of the S-O bond to produce ground state products. The minor SO + CH3 channel has two components in comparable proportions: a fast feature corresponding to rapid C-S cleavage on the excited state to produce CH3 and electronically excited SO, and a slow feature due to internal conversion of CH3SO followed by statistical dissociation on the ground electronic state. Statistical ground state dissociation also produces small amounts of CH2SO, likely sulfine, and H-atoms.

Fast beam photofragment translational spectroscopy of the phenoxy radical at 225 nm, 290 nm, and 533 nm.

Photodissociation of the phenoxy radical (C6H5O) is investigated using fast beam photofragment translational spectroscopy. Phenoxy radicals are generated through photodetachment of phenoxide anions (C6H5O-) at 532 nm. Following photoexcitation of the radicals at 225 nm (5.51 eV), 290 nm (4.27 eV), or 533 nm (2.33 eV), photofragments are collected in coincidence to determine their masses, translational energy, and scattering angle for each dissociation event. Two-body dissociation yields exclusively CO + C5H5, and three-body dissociation to CO + C2H2 + C3H3 and CO + C5H4 + H is also seen at the two higher energies. The translational energy distributions for two-body dissociation suggest that dissociation occurs via internal conversion to the ground electronic state followed by statistical dissociation. The absorption of an additional 532 nm photon in the photodetachment region provides some C6H5O radicals with an additional 2.33 eV of energy, leading to much of the two-body dissociation observed at 533 nm and the three-body dissociation at the two higher excitation energies.

Photodissociation dynamics of the simplest alkyl peroxy radicals, CH3OO and C2H5OO, at 248 nm.

The photodissociation dynamics of the simplest alkyl peroxy radicals, methyl peroxy (CH3OO) and ethyl peroxy (C2H5OO), are investigated using fast beam photofragment translational spectroscopy. A fast beam of CH3OO- or C2H5OO- anions is photodetached to generate neutral radicals that are subsequently dissociated using 248 nm photons. The coincident detection of the photofragment positions and arrival times allows for the determination of mass, translational energy, and angular distributions for both two-body and three-body dissociation events. CH3OO exhibits repulsive O loss resulting in the formation of O(1D) + CH3O with high translational energy release. Minor two-body channels leading to OH + CH2O and CH3O + O(3P) formation are also detected. In addition, small amounts of H + O(3P) + CH2O are observed and attributed to O loss followed by CH3O dissociation. C2H5OO exhibits more complex dissociation dynamics, in which O loss and OH loss occur in roughly equivalent amounts with O(1D) formed as the dominant O atom electronic state via dissociation on a repulsive surface. Minor two-body channels leading to the formation of O2 + C2H5 and HO2 + C2H4 are also observed and attributed to a ground state dissociation pathway following internal conversion. Additionally, C2H5OO dissociation yields a three-body product channel, CH3 + O(3P) + CH2O, for which the proposed mechanism is repulsive O loss followed by the dissociation of C2H5O over a barrier. These results are compared to a recent study of tert-butyl peroxy (t-BuOO) in which 248 nm excitation results in three-body dissociation and ground state two-body dissociation but no O(1D) production.

Production and Photodissociation of the Methyl Perthiyl Radical.

The photodissociation dynamics of the methyl perthiyl (CH3SS) radical are investigated via molecular beam photofragment translational spectroscopy, using "soft" electron ionization to detect the radicals and their photofragments. With this new capability, we have shown that CH3SS can be generated from flash pyrolysis of dimethyl trisulfide. Utilizing this source of radicals and the advantages afforded by soft electron ionization, we have reinvestigated the photodissociation dynamics of CH3SS at 248 nm, finding CH3S + S to be the dominant dissociation channel with CH3 + SS as a minor process. These results differ from previous work reported in our laboratory in which we found CH3 + SS and CH2S + SH as the main dissociation channels. The difference in results is discussed in light of our new capabilities for characterization of radical production.

Three-body photodissociation dynamics of [formula omitted](CO 2)

The three-body dissociation of I 2 - (CO 2) following excitation of the I 2 - chromophore to the repulsive A′ 2Π g,1/2 and B 2 Σ g , 1 / 2 + electronic states at 1.72 and 3.21 eV has been investigated using fast beam photofragment translational spectroscopy. The translational energy distributions for three-body dissociation provide a direct measurement of the CO 2 binding energy, yielding a value of 218 ± 10 meV. These distributions are vibrationally resolved and show that some CO 2 is produced with bend excitation. Dalitz plots show that the dominant three-body decay mechanism is asynchronous–concerted decay, in which the two bond cleavages are distinct but nearly simultaneous events.

Two- and three-body photodissociation dynamics of diiodobromide (I2Br−) anion

The photodissociation of gas-phase I(2)Br(-) was investigated using fast beam photofragment translational spectroscopy. Anions were photodissociated from 300 to 270 nm (4.13-4.59 eV) and the recoiling photofragments were detected in coincidence by a time- and position-sensitive detector. Both two- and three-body channels were observed throughout the energy range probed. Analysis of the two-body dissociation showed evidence for four distinct channels: Br(-) + I(2), I(-) + IBr, Br+I(2) (-), and I + IBr(-). In three-body dissociation, Br((2)P(3∕2)) + I((2)P(3∕2)) + I(-) and Br(-) + I((2)P(3∕2)) + I((2)P(3∕2)) were produced primarily from a concerted decay mechanism. A sequential decay mechanism was also observed and attributed to Br(-)((1)S)+I(2)(B(3)Π(0u) (+)) followed by predissociation of I(2)(B).

Photodissociation of the propargyl and propynyl (C3D3) radicals at 248 and 193 nm

The photodissociation of perdeuterated propargyl (D(2)CCCD) and propynyl (D(3)CCC) radicals was investigated using fast beam photofragment translational spectroscopy. Radicals were produced from their respective anions by photodetachment at 540 and 450 nm (below and above the electron affinity of propynyl). The radicals were then photodissociated at 248 or 193 nm. The recoiling photofragments were detected in coincidence with a time- and position-sensitive detector. Three channels were observed: D(2) loss, CD+C(2)D(2), and CD(3)+C(2). Observation of the D loss channel was incompatible with this experiment and was not attempted. Our translational energy distributions for D(2) loss peaked at nonzero translational energy, consistent with ground state dissociation over small (<1 eV) exit barriers with respect to separated products. Translational energy distributions for the two heavy channels peaked near zero kinetic energy, indicating dissociation on the ground state in the absence of exit barriers.

D atom loss in the photodissociation of the DNCN radical: Implications for prompt NO formation

The photodissociation of DNCN following excitation of the C 2A"<--X 2A" electronic transition was studied using fast beam photofragment translational spectroscopy. Analysis of the time-of-flight distributions reveals a photodissociation channel leading to D+NCN competitive with the previously observed CD+N2 product channel. The translational energy distributions describing the D+NCN channel are peaked at low energy, consistent with internal conversion to the ground state followed by statistical decay and the absence of an exit barrier. The results suggest a relatively facile pathway for the reaction CH+N2-->H+NCN that proceeds through the HNCN intermediate and support a recently proposed mechanism for prompt NO production in flames.

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.

Photodissociation spectroscopy and dynamics of the CH2CFO radical.

The photodissociation spectroscopy and dynamics resulting from excitation of the B (2)A(")<--X (2)A(") transition of CH(2)CFO 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 approximately 6000 cm(-1) higher in energy than previously reported in a laser-induced fluorescence excitation spectrum. At all photon energies investigated, only the CH(2)F+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 CH(2)F+CO products occurs on the ground state potential energy surface with the isomerization barrier between CH(2)CFO and CH(2)FCO governing the observed translational energy distributions.

Two- and three-body photodissociation of gas phase I(3) (-).

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 < or = 3.87 eV, two-body dissociation that generates I- + I2(A 3Pi1) and vibrationally excited I2- (X 2Sigmau+) + I(2P(3/2)) is observed, while at energies > or = 3.87 eV, I*(2P(1/2)) + I2- (X 2Sigmau+) is the primary two-body dissociation channel. In addition, three-body dissociation yielding I- +2I(2P(3/2)) 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 spectroscopy and dynamics of Si4

The photodissociation of Si4 has been investigated using fast beam photofragment translational spectroscopy. The photofragment yield (PFY) spectrum shows features between 21370 and 22220 cm−1 corresponding to the 1B1u←X̃1Ag transition that are attributed to multi-photon dissociation. Single-photon dissociation was examined at higher excitation energies ranging from 5.17 to 6.42 eV. The dominant product channel was found to be Si3+Si. Experimental photofragment translational energy distributions were modeled by phase space theory (PST), indicating statistical photodissociation with no exit barrier along the dissociation coordinate. PST modeling yields a dissociation energy of 4.60±0.15eV and ΔHf,00(Si4)=6.75±0.24eV.

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 B̃ 2Π←X̃ 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 B̃ 2Π←X̃ 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. While conclusive evidence for the higher energy O(3P)+CN(X 2Σ+) channel was not observed, the presence of this dissociation pathway could not be excluded.

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 B̃ 2A′←X̃ 2A″ band and a higher-energy band system assigned to the C̃ 2A″←X̃ 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.

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 P3/22 or P1/22 state. The 1:1 channel corresponds primarily to concerted three-body dissociation to three I atoms.

Photodissociation dynamics of the CNN free radical

The spectroscopy and photodissociation dynamics of the à 3Π and B̃ 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 à 3Π←X̃ 3Σ− origin were found to predissociate. Photofragment yield spectra for the B̃ 3Σ−←X̃ 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 à 3Π and B̃ 3Σ− states. The translational energy distributions for the à 3Π state are bimodal with high and low translational energy components. The distributions for the B̃ 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 dissociation mechanism and provide more accurate values: ΔfH0(CNN)=6.16±0.05 eV and ΔfH298(CNN)=6.15±0.05 eV. These values, coupled with recent D0(RH) and D298(RH) values from Clifford et al. [J. Phys. Chem. 102, 7100 (1998)], yield ΔfH0(HCNN)=5.02±0.18 eV, ΔfH298(HCNN)=4.98±0.18 eV, ΔfH0(H2CNN)=3.09±0.21 eV, and ΔfH0(H2CNN)=3.09±0.21 eV.

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 of Linear Carbon Clusters Cn (n = 4−6)

The photodissociation of mass-selected linear carbon clusters (Cn, n = 4−6) is studied using fast beam photofragment translational spectroscopy. The photofragment yield (PFY) spectra consist of several continua spanning the whole visible and ultraviolet region. The product mass distributions for dissociation of Cn clusters are dominated by C3 and its partner fragment Cn-3, although some minor channels are also identified for dissociation of C4 and C5 clusters. Translational energy P(ET) distributions for the C3 + Cn-3 channel were measured at several photolysis energies. The PFY spectra and P(ET) distributions indicate that multiphoton dissociation occurs at photon energies below the dissociation threshold and that both single-photon and multiphoton dissociation occur above the threshold. The one-photon components of the P(ET) distributions can be modeled by phase space theory (PST), suggesting that photoexcitation is followed by internal conversion to the ground state. The PST analysis yields dissociatio...

Photodissociation dynamics of the singlet and triplet states of the NCN radical

The spectroscopy and photodissociation dynamics of the NCN radical have been investigated by fast beam photofragment translational spectroscopy. The B̃ 3Σu−←X̃ 3Σg−, c̃ 1Πu←ã 1Δg, and d̃ 1Δu←ã 1Δg transitions were examined. The major dissociation products for the B̃ 3Σu− and c̃ 1Πu states are N2(X̃ 1Σg+)+C(3P), while the d̃ 1Δu state dissociates to N2(X̃ 1Σg+)+C(1D). The dissociation channel, N(4S)+CN(X̃ 2Σ+) is observed for the B̃ 3Σu− state at photon energies greater than 4.9 eV, where it comprises ≈25±10% of the total signal. At all photon energies, the photofragment translational energy distributions show a resolved progression corresponding to the vibrational excitation of the N2 photofragment. The rotational distributions of the molecular fragments suggest that the dissociation pathway for the N2 loss channel involves a bent transition state while the N+CN photofragments are produced via a linear dissociation mechanism. The P(ET) distributions provide bond dissociation energies of 2.54±0.030 and 4.56±0.040 eV for the N2 and CN loss channels, respectively, yielding ΔHf,0 K(NCN)=4.83±0.030 eV.