Vacuum Ultraviolet Photodissociation Research Articles

Production of O(1D) and O(3P) by vacuum ultraviolet photodissociation of molecular oxygen

A pulsed fluorine excimer laser operating at 1576 Å wavelength, 10 ns pulse width, 10 Hz repetition rate, and ∼0.2 MW average output power is used to photodissociate ground state molecular oxygen, O2(X3∑−g). The dissociation products, O(1D) and O(3P), are produced with unit quantum efficiency and ∼0.4 eV translation energy. Time-resolved techniques are used to monitor the O(1D2)→O(3P2) radiative transition over the O2 pressure range 10–400 mTorr. The O(1D)–O2 quenching rate constant is determined to be (3.7±0.3)×10−11 cm3 s−1. The O(1D)/O2 dissociation fraction is estimated to be 0.4% to ∼50% corresponding to O(1D) densities as high as 3×1015 cm−3.

Rotationally resolved photofragment alignment and the vacuum ultraviolet photodissociation of H 2O

The photodissociation of H 2O in the vacuum UV (λ < 135 nm) generates rotationally aligned populations of OH(A 2ε +) fragments. Measurements of their rotationally resolved fluorescence polarisation following photolysis at 130.4 nm reinforces the suggestion that the vibrational structure in the B̃ 1A 1←X̃ 1A 1 continuum is associated with long-lived “resonance” in the excited state. The low levels of alignment at 121.6 nm reflect a delay in the radiationless transfer out of the quasi-bound D̃ 1A 1 Rydberg state initially populated.

NO fluorescence during vacuum-ultraviolet photodissociation of CFCl2NO

Gas-phase photodissociation of CFCl2NO at 147 nm results in the formation of electronically excited NO in the B2Π state, giving rise to the well known NO β-emission bands. Collisional effects consequent upon the addition of helium lead to the appearance of fluorescence from the NO(A2∑+) state with emission from the ν′= 2 and 3 levels dominating the spectrum. The results are compared with data obtained previously for other nitrosocompounds.

Vacuum ultraviolet laser-induced photodissociation of cyanogen iodide. Cyanogen ~B2.SIGMA.+ state population and kinetics

Vacuum ultraviolet laser-induced photodissociation of cyanogen iodide. Cyanogen ~B2.SIGMA.+ state population and kinetics

Vacuum ultraviolet photodissociation of ammonia

New measurements of the photofragment fluorescence excitation spectra of NH2,ND2(Ã2A1) and NH,ND(c1Π) produced through the vacuum u.v. photodissociation of NH3,ND3 are reported. The spectra, which have been recorded at much higher resolution than previously, eliminate controversy concerning the predissociation of the B and C states and identify a contribution from the totally symmetric stretching mode in the vibronic structure. Measurements of rotational energy disposal in the NH(c1Π) fragment are consistent with a photodissociation threshold at 136.5 ± 0.5 nm and a revised value for ΔH⊖f0(NH)= 322.6 ± 7.1 kJ mol–1.

Photodissociation of methylnitrite in the vacuum ultraviolet. I. Identification and quantum yields of electronically excited NO products

The fluorescence of the photofragments resulting from vacuum ultraviolet photodissociation of methyl nitrite (CH3ONO) in the gas phase has been studied using synchrotron radiation from Orsay Electron Storage Ring (ACO) as the source of excitation. In the spectral region between 1100 and 1600 Å where CH3ONO shows a diffuse absorption spectrum, the formation of NO in A 2Σ+ (v′=0,1,2), C 2Π and D 2Σ+ has been identified by time and energy-resolved spectra. The quantum yields of the photodissociation channels leading to the NO (A,C,D) states have been determined by comparison of the emission intensity with that of pure NO directly excited in A 2Σ+, C 2Π, v′=0, D 2Σ+, v′=0: φ (CH3ONO→NO A)=0.18±0.03 at λexc=1440 Å; φ (CH3ONO→NO C, v′=0)=0.07±0.02 at λexc=1380 Å; and φ (CH3ONO→NO D, v′=0)=0.05±0.02 at λexc=1200 Å.

Energy disposal in the vacuum ultraviolet photodissociation of CS2 and CSe2

Vibrational and rotational energy disposal in CS(A1Π), produced through monochromatic vacuum u.v. photodissociation of CS2, has been studied under effectively collision-free conditions and at considerably higher resolution than in earlier studies by Lee and Judge and Okabe. Vibrational energy disposal in CSe(D1Π) produced from CSe2 has also been studied under similar conditions. The results, which indicate the importance of Franck–Condon considerations, are discussed qualitatively in terms of the formalism developed by Morse et al. Estimates of the overall transition moments, Rv′v″, and Franck–Condon factors for CS(A→X) are presented which confirm recent calculations by Coxon et al. and measurements by Carlson et al. and necessitate a revision of earlier data obtained under lower resolution by Lee and Judge.

NO and CF2 fluorescence during vacuum ultraviolet photodissociation of CF2ClNO

Photolysis of CF2ClNO at 147 and 123 nm results in electronically excited fragments, fluorescence from which is observed. At 147 nm NO (B2Π) is produced, giving rise to the well known NO β-bands, contrasting with previous studies on CF3NO, in which NO (A2∑+) dominated. Addition of non-reactive gases (e.g., He) results in quenching of fluorescence from the higher vibronic levels of the NO (B2Π) state, while at the same time fluorescence from NO(A2∑+) becomes increasingly important. Photolysis at 123 nm give a very different emission spectrum, the most important contributor being the (1B1) state of CF2. Ionization also occurs at 123 nm (10.03 eV) with a quantum yield of ca. 0.08. These results are compared and contrasted with data obtained previously for CF3NO.

Vacuum ultraviolet photodissociation spectroscopy of CH3CN, CD3CN, CF3CN and CH3NC

The results obtanined from a study of the vibrational and rotational energy disposals in the vacuum ultraviolet photodissociation of CH3CN, CD3CN, CF3CN and CH3NC have been explained in terms of a statistical partitioning (consistent with dissociation from a long-lived intermediate via a “loose” transition state) together with the restrictions imposed by conservation of overall energy and angular momentum. The vacuum ultraviolet absorption spectra of the four molecules have been assigned in an attempt to identify the character of the photoexcited states at each photolysis wavelength. The observed distributions are in marked contrast to those obtained in the photodissociation of the simpler cyanogen halides from comparable excited states.The following bond dissociation energies were obtained through an analysis of the rotational energy disposal, D(CH3—CN)= 517 ± 3, D(CD3—CN)= 515 ± 3, D(CF3—CN)= 473 ± 3, D(CH3—NC)= 430 ± 8 Kj MOL –1, to yeld ΔH°f0 values in good agreement with previous measurments.

Electronically excited nitric oxide by vacuum ultraviolet photodissociation of nitroso-compounds and nitrites

The gas-phase photolysis of CF3NO and EtONO at 123 and 147 nm and of ButNO at 147 nm gives rise to electronically excited nitric oxide which fluoresces. The strongest emission is that of the A2∑+–X2II transition (γ-bands), although other transitions are also observed. The vibrational level populations in the NO(A2∑+) state show some correlation with the energy of the incident radiation and the strength of the R–NO bond. Addition of helium to the CF3NO or EtONO produces an enhancement of emission from the v′= 2 and 3 levels of the A2∑+ state, while emission from v′= 0 and 1 is almost unchanged. Addition of low pressures of N2 and H2 also gives some enhancement of v′= 2 and 3. These observations are discussed in terms of the possible excited states of NO produced during the photolysis, and the deactivating effects of the added gas. Photolysis of ONCN at 147 nm yields electronically excited cyanide radicals and the (B2∑–X2∑) violet bands are observed. There is evidence for considerable rotational excitation of the CN radical, an effect well known in the vacuum u.v. photolysis of other cyanides, e.g. ICN.

Energy partitioning in photodissociation. Systematic study of rotational energy disposal in vacuum ultraviolet photodissociation of the cyanogen halides and HCN

Rotational energy disposal in CN(B2Σ+)v= 0 produced through photodissociation of HCN, ClCN, BrCN and ICN has been measured under collision-free conditions and at high resolution over a wide range of absorbed wavelengths in the vacuum u.v. The experimental measurements include systematic variation of (i) the mass of the departing atom, (ii) the electronic character of the initially excited state, (iii) the magnitude of the available excess energy and (iv) the level of excitation of the bending mode in HCN (C1A′). The results have been discussed in terms of Franck–Condon considerations and the constraints introduced by geometric changes on photo-excitation, and the requirements of energy and angular momentum conservation following photodissociation.

CF2 emission during vacuum ultraviolet photodissociation of CF2Br2

Photolysis of CF2Br2 at wavelengths shorter than 135 nm leads to the production of electronically excited CF2. At low pressure fluorescence is observed over the range 230–340 nm. Considerable vibrational excitation of the CF2 is evident. The CF2 fluorescence is quenched by H2, D2, CO and CO2 with rate constants of 5.6 × 109, 2.7 × 109, 4 × 1010 and 3.2 × 1010 dm3 mol–1 s–1, respectively.

Polarized photofluorescence excitation spectroscopy: H 2O revisited and the vacuum-ultraviolet photodissociation of D 2O

The fluorescence excitation spectra of H 2O and D 2O have been recorded in the vacuum ultraviolet region, and the technique of polarized photofluorescence excitation spectroscopy has been applied to the photodissociations H 2O, D 2O + hv(gl > 115 nm) → H, D + OH, OD (A 2Σ +). In the case of H 2O, results obtained with major experimental improvements have necessitated the reappraisal of earlier polarization measurements, and some conclusions previously reached have been corrected. For both H 2O and D 2O, symmetry assignments based on the absorption spectra have been confirmed and, in addition, our results indicate a contribution from an electronic state not hitherto identified in optical spectroscopy. Polarization measurements for D 2O suggest that the excited states populated at λ > 126 nm are longer-lived than the corresponding states in H 2O.

Polarised photofluorescence excitation spectroscopy. Part 2.—Vacuum ultraviolet photodissociation of HCN and BrCN

The new technique of polarised photofluorescence excitation spectroscopy has been applied to the vacuum u.v. photodissociations HCN, BrCN [graphic omitted] H, Br + CN(B2∑+) and a theoretical analysis of the technique has been developed. By monitoring the polarised excitation spectrum of the fluorescent and rotationally excited CN fragments the following conclusions have been reached.(i) The fluorescence excitation spectra closely resemble the absorption spectra.(ii) The primary excited states of HCN and BrCN that predissociate to form CN(B2∑+) are all non-linear and have the symmetry A′: transitions into the predissociating levels of HCN(C1A′) are predominantly type B, while those into the “B” and “C” states of BrCN are type A.(iii) The predissociative lifetimes of the “B” and “C” states of BrCN are ≲ 0.6 ps, and vibronic levels in which the C—Br stretching mode is excited are shorter lived than those where the C—N mode is excited.(iv) The slight non-linearity of the predissociated Rydberg states in BrCN lying at λ > 125 nm may be caused by a Renner–Teller interaction.

Vacuum ultra-violet photodissociation of alkyl iodides. Kinetics of the reaction H + ICH3? HI + CH3

The vacuum u.v. photodissociation of alkyl iodides has been investigated using flash photolysis techniques. At wavelengths between 140 and 170 nm, the primary process is selective and results in C—H rather than C—I scission: it is also sensitive to deuterium isotope substitution. At wavelengths > 170 nm, the quantum yield of C—H scission is < 10–2. The selective C—H scission has allowed an investigation of energy disposal in the reactions H + ICH3→ HI + CH3(3a) D + ICD3→ DI + CD3(3b) as well as their kinetics. The rate constant k3a has been measured as (5.86± 0.29)× 1012 cm3 mol–1 s–1 at (293 ± 2)K, by monitoring the rate of development of the Rydberg absorption bands of HI. No evidence of transient vibrational (or electronic) excitation of the methyl radicals produced in (3) or through the photodissociation of methyl iodide at > 170 nm, has been observed. These, and related observations, can be understood if, in photodissociation or bimolecular iodine atom transfer, the methyl radical attains a near planar conformation before recoiling from the neighbouring iodine atom.