Dadong Xu

Ultraviolet photodissociation of bromoform at 234 and 267 nm by means of ion velocity imaging

The photochemistry of bromoform is of considerable importance to understanding the impact of short-lived halogen species on bromine chemistry in the atmosphere. In the present work, the products of the ultraviolet photodissociation of bromoform at 234 and 267 nm are determined by time-of-flight mass spectrometry and velocity ion imaging. Both ground Br (2P3/2) and spin–orbit excited Br (2P1/2) atoms are found to be formed via resonance-enhanced multiphoton ionization detection. Radical products are detected via vacuum ultraviolet photoionization at 118 nm. The results indicate that there is a primary molecule bromine elimination channel consisting of CHBr+Br2. The quantum yields for atomic Br and molecular Br2 elimination channels are determined from the time-of-flight spectra to be 0.74 and 0.26 at 234 nm, respectively. At 267 nm, they are 0.84 and 0.16, respectively. Energy and angular distributions are deduced from the 2D images of Br, CHBr, and CHBr2. The direct studies described in this paper on the ...

Reinvestigation of CS2 dissociation at 193 nm by means of product state-selective vacuum ultraviolet laser ionization and velocity imaging.

A branching ratio of 1.6 +/- 0.3 for S(3P)/S(1D) is obtained for the dissociation of CS2 with very low fluence 193 nm laser (less than 2 mJ/cm2), in which the S(3P) and S(1D) have been state-selectively ionized using VUV lasers at different wavelengths. The anisotropy parameters betamax(3P) = 0.8 and betamax(1D) = 1.9 indicate that these channels are preferentially populated at different geometries and the lifetime is very short.

Photodissociation of the dibromomethane cation at 355 nm by means of ion velocity imaging

The photodissociation dynamics of the dibromomethane cation, CH2Br2+, have been studied by means of ion velocity imaging and time-of-flight mass spectroscopy methods at 355 nm. The dibromomethane cation is produced through the direct ionization of the neutral molecule with a pulsed 118 nm laser. The translational energy distribution shows that the CH2Br+ fragment is formed in highly vibrationally excited states with two distinguished dissociation channels following a parallel excitation from 2b2 to 3b2 of the parent ion. The broad fast speed distribution is fit with two Gaussian functions, from which a branching ratio of Br*(2P1/2) to Br(2P3/2) is determined as 2.2:1. The sharp peak with very slow speed was modeled with a Boltzmann distribution with a temperature of 300 K. This channel contributes ∼4.5% to the reaction and is proposed to proceed on the ground state surface following internal conversion. Ab initio calculations for both the parent and the fragment ions have been performed that strongly supp...

Branching ratios of C2 products in the photodissociation of C2H at 193 nm

Experimental and theoretical results are combined to show that vibrationally excited C2H radicals undergo photodissociation to produce C2 radicals mainly in the B 1Δg state. Infrared (IR) emissions from the photolysis of acetylene with a focused and unfocused 193 nm excimer laser have been investigated using step-scan Fourier transform infrared (FTIR) emission spectroscopy at both low and high resolution. With an unfocused laser, the low-resolution infrared emission spectra from the C2H radicals show a few new vibrational bands in addition to those previously reported. When the laser is focused, the only emissions observed in the 2800–5400 cm−1 region come from the electronic transitions of the C2 radicals. Most of the emissions are the result of the B 1Δg→A 1Πu transition of C2 although there are some contributions from the Ballik–Ramsay bands C2(b 3Σg−→a 3Πu). A ratio of [B 1Δg]/[b 3Σg−]=6.6 has been calculated from these results. High quality theoretical calculations have been carried out to determine ...

Photodissociation of C2H at 193 nm: Branching ratios for the formation of C2 in the X 1Σg+, A 1Πu, and B′ 1Σg+ states

The ratio of the nascent population of C2 (X 1Σg+) to C2 (A 1Πu) to C2 (B′ 1Σg+) produced from the photodissociation of C2H at 193 nm has been measured using laser induced fluorescence. This technique is typically used to measure rotational and vibrational distributions in a given electronic state. Here, we have extended the technique to measure the relative electronic distributions in the C2 photofragment. From the simultaneous measurement of the Mulliken (X 1Σg+–D 1Σu+) and Freymark (A 1Πu–E 1Σg+) systems, the nascent population ratio of C2 (A 1Πu) to C2 (X 1Σg+) molecules was determined. Similarly, from the measurement of the Deslandres–D’Azumbuja (A 1Πu–C 1Πg) and the LeBlanc (B′ 1Σg+–D 1Σu+) systems, the nascent population ratio of C2 (A 1Πu) to C2 (B′ 1Σg+) was determined. The overall ratio for the production of C2 in the X:A:B′ electronic states was found to be 1:19:1.4. These results along with the results of high quality ab initio calculations of Cui and Morokuma (unpublished) are used to discuss...

One photon dissociation and multiphoton dissociative ionization of bromochlorodifluoromethane (CF2BrCl) at 267 nm region

The photodissociation dynamics of CF2BrCl have been studied near 267 nm using time-of-flight mass spectrometry combined with ion velocity imaging. The bromine atoms are probed via 2+1 resonance enhanced multiphoton ionization. The one-photon dissociation channels of CF2BrCl produce bromine atoms in both ground state Br(2P3/2) and excited state Br(2P1/2). Translational energy and angular distributions for each channel are derived from the two-dimensional images of both of the bromine atoms using the back-projection method. The time of flight mass spectra indicate that the CF2Cl+ ions are also formed with the focused laser in the 259–270 nm region. Six images were taken in this wavelength region and they show the typical characteristics recently reported by several groups that are attributed to dissociative ionization arising from multiphoton absorption. The changes in the energy and angular distributions of CF2Cl+ as a function of wavelength are described in terms of a resonance enhanced multiphoton dissoc...

Photodissociation of the acetone cation at 355 nm using the velocity imaging technique

Photodissociation of acetone cations, CH3COCH3+, at 355 nm has been studied by means of the ion velocity imaging technique. Acetone cations are produced via direct photoionization of a supersonic beam of acetone at 118 nm generated by frequency tripling the 355 nm laser. Only the acetyl cation, CH3CO+, could be detected as a dissociation product in the time-of-flight mass spectrometer. The acetyl ion signal depends upon the fifth power of the 355 nm laser energy, while the acetone ion signal depends upon the third power. This suggests that the fragment ion is produced via two-photon absorption of 355 nm photons by the acetone cation. The total translational energy distribution and angular distribution of acetyl cation were derived from the 2D images of CH3CO+ for the reaction CH3COCH3++2hν355 nm→CH3CO++CH3*. The translational energy distribution suggests that methyl radicals are produced in two electronically excited states, the Rydberg 3s 1 2A1′ and the valence 1 2A″ states. The anisotropy parameter β sh...

Velocity map imaging studies of the Lyman α photodissociation mechanism for H atom production from hydrocarbons

H atoms produced in Lyman α photolysis of ethane, propane, and ethylene have been studied using velocity map imaging techniques. Two types of H atoms are identified, one formed along with an alkyl radical in the Rydberg state and the other by the subsequent decomposition of this Rydberg radical.

Using the Ultraviolet and Visible spectrum of Comet 122P/de Vico to Identify the Parent Molecule CS2

A comparison has been made between the unidentified spectral lines of comet 122P/de Vico and observed dispersed fluorescence spectra in the UV and visible region from jet cooled CS2. Twenty-eight lines of the spectral atlas of this comet have been identified as originating from emission of CS2 when this molecule is excited by solar radiation in the near-ultraviolet region of the spectra. This work proves the presence of CS2 in comets and points to a new method for identifying parent molecules in comets.

Photodissociation of bromoform cation at 308, 355, and 610 nm by means of time-of-flight mass spectroscopy and ion velocity imaging

The photodissociation dynamics of bromoform cation, CHBr3+, have been studied at 308, 355, and 610 nm by means of time-of-flight mass spectrometry combined with ion velocity imaging. The bromoform cation is produced via vacuum ultraviolet photoionization at 118 nm. The only fragment ions found in the time-of-flight spectra at 355 and 610 nm are CHBr2+ ions. At 308 nm CBr+ and CHBr+ ions are also found in the time-of-flight spectrum in addition to the CHBr2+ ions. These results indicate that there is only one dissociation channel that produces CHBr2++Br at both 355 and 610 nm, while there are two more channels involved at 308 nm. One is a molecular bromine elimination channel forming CHBr++Br2, and the other is a three-body dissociation channel producing CBr++HBr+Br. Translational energy and angular distributions for each channel are deduced from the two-dimensional images of CHBr2+, CHBr+, and CBr+. A soft fragment impulsive model closely reproduces the fraction of the available energy in translation for ...