Photolysis Wavelength Research Articles

Time-dependent wave packet calculations for the photodissociation of molecular chlorine

The dissociation processes of Cl 2 molecule have been studied at numerous photolysis wavelengths (310–470 nm) using time-dependent wave packet method. The initial wave packets are propagated on the excited state potentials utilizing the splitting operator technique. The optical cross-sections are calculated numerically by extracting the dynamics information at a large internuclear separation. Applying the Rozen–Zener–Demkov model, the radial non-adiabatic transition probabilities from C 1Π u to 1 u(III) electronic state are predicted, the branching ratio of various product channels and anisotropy parameter β(Cl ∗) are determined.

Quantum yield of Cl* (2P1/2) production in the gas phase photolysis of CCl4 in the ultraviolet

In this paper, we have probed the dynamics of chlorine atom production from the gas phase photodissociation of carbon tetrachloride at 222 and 235 nm. The quantum yield, φ* of Cl* (2P1/2) production has been determined by probing the nascent concentrations of both excited (2P1/2) and ground state (2P3/2) chlorine atoms by suitable resonance-enhanced multiphoton ionization (REMPI) detection schemes. Although at the photolysis wavelengths the absorption of carbon tetrachloride is weak, significant amounts of Cl* are produced. Surprisingly, the quantum yield of Cl* production does not follow the absorption spectrum closely, which gives rise to the possibility of an indirect dissociation mechanism present in CCl4 along with direct dissociation at these ultraviolet wavelengths

TR-LII for sizing of carbon particles forming at room temperature

Time-resolved laser-induced incandescence (TR-LII) was applied for the determination of particle sizes during carbon-particle formation from supersaturated atomic carbon vapor that was generated by laser photolysis of carbon suboxide (C3O2) at room temperature. Thus, the solid carbon particles were formed under hydrogen-free conditions. The TR-LII technique was used for in situ size measurement of growing carbon particles and samples of final particles were analyzed by transmission electron microscopy (TEM). It was found that the particles grow to a final size of 4–12 nm within 0.02–1 ms. The properties of the obtained particles depend on the initial conditions in the reaction volume, i.e. concentration of carbon suboxide, pressure and type of gas diluter, photolysis wavelength, and laser pulse energy. The comparison of TR-LII and TEM particle sizing results yields information about the effective thermal energy accommodation coefficients for He, Ar, CO, and C3O2 molecules on carbon particles.

3‐Nitro‐2‐naphthalenemethanol: A Photocleavable Protecting Group for Carboxylic Acids.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Imaging CIN3 photodissociation from 234 to 280 nm

We report Cl((2)P(3/2)) and Cl*((2)P(1/2)) fragment images following ClN(3) photolysis in the 234-280 nm region measured by velocity map imaging. Kinetic energy distributions change shape with photolysis wavelength from bimodal at 234 and 240 nm to single peak at 266 and 280 nm. Where two peaks exist, their ratio is significantly different for Cl and Cl* fragments. The single peak of 266 and 280 nm and the faster peak at 234 and 240 nm are assigned to a Cl + linear-N(3) dissociation channel, in agreement with previous work. The slow peak in the bimodal distributions is assigned to the formation of a high energy form (HEF) of N(3). Candidates for the identity of HEF-N(3) are discussed. Combining our data with photofragmentation translational spectroscopy results, we determined the threshold for the appearance of HEF-N(3) at 4.83 +/- 0.17 eV photolysis energy. This threshold behavior is similar to recently reported results on the wavelength dependence of HN(3) photolysis, where the threshold was associated with a ring closed isomer of HN(3) on the S(1) potential energy surface. We also note that the HEF-N(3) formation threshold observed for ClN(3) occurs where the energy available to the products equals the isomerization barrier from linear to cyclic-N(3).

Determination of absolute photoionization cross sections of the phenyl radical

Photoionization cross sections of the phenyl radical to form the phenyl cation were measured using tunable vacuum ultraviolet synchrotron radiation coupled with photofragment translational spectroscopy. The phenyl radical was produced via 193- or 248-nm dissociation of chlorobenzene. At 10.0 eV, the photoionization cross sections for the phenyl radical averaged over product channels were found to be 13.4 +/- 2.0 and 13.2 +/- 2.0 Mb, respectively, with very little effect seen from the range of internal excitation produced at the two photolysis wavelengths. Using the photoionization cross section values for each channel, photoionization efficiency curves for the phenyl radical were placed on an absolute scale from 7.8 to 10.8 eV.

Photodissociation Dynamics of <i>n</i>-C<sub>7</sub>H<sub>15</sub>Br Molecule by Ion Velocity Imaging

Photodissociation dynamics of n-C7H15Br molecule was investigated at different wavelengths of 231 to 239 run by an ion imaging spectrometer operated under optimal conditions for velocity mapping, where the ions were generated from (2+1) resonance enhanced ionization (REMPI) of the photofragments Br(P-2(3/2)) and Br*(P-2(1/2)) at the same wavelength as that of the photolysis laser. The released kinetic energies and the angular distributions of both fragments at different photolysis wavelengths were derived from the three-dimensional velocity images. The probabilities of the individual pathways of the fragments at 234 nm were calculated from the relative quantum yield and angular distribution. The total translational energy distributions were interpreted using soft impulsive model. The anisotropy parameter for Br* is sensitive to the photolysis wavelength. This fact can be explained by the adiabatic and non-adiabatic dissociation processes in the electronically excited states of n-C7H15Br.

Vacuum ultraviolet photodissociation of SH radical at 121.6 nm

Vacuum ultraviolet (VUV) photodissociation of jet-cooled SH radical is studied at the photolysis wavelength of 121.6 nm using high- n Rydberg atom time-of-flight (HRTOF) technique. Both the ground-state S( 3P J ) and the excited-state S( 1D 2) and S( 1S 0) products are observed, with a branching fraction of S( 3P J ):S( 1D 2):S( 1S 0) = 0.07 ± 0.01:0.82 ± 0.01:0.11 ± 0.01. The spin–orbit branching fraction of the minor S( 3P J ) product is S( 3P 2):S( 3P 1):S( 3P 0) = 0.28 ± 0.12:0.54 ± 0.16:0.18 ± 0.09. The predominant production of S( 1D 2) is consistent with a direct dissociation from the repulsive 2 2Π state. The S( 3P J ) product fine-structure state distribution from SH photodissociation at 121.6 nm is significantly different from that in the UV region.

Photodissociation of state selected BrCl+ cations: branching ratios and angular anisotropies of the Br+ product forming channels

High resolution velocity map ion imaging methods have been used to explore the photofragmentation dynamics of state-selected BrCl+(X2ΠΩ, v=0) cations following excitation in the wavelength range 370–420 nm. The parent ions are formed by 2 + 1 resonance enhanced multiphoton ionization (REMPI) via the [2Π1/2]5sσ, v=0 Rydberg level. Experimental measurables include the wavelength dependent branching ratios and recoil anisotropies of the various Br+(3 P J ) + Cl(2 P J ) product channels. Ground state Br+(3 P 2) fragments dominate the fragment ion yield throughout the photolysis wavelength range investigated, with spin–orbit excited Cl(2 P 1/2) atoms the preferred co-fragment whenever energetically allowed. All fragmentation channels show a predominantly parallel recoil anisotropy, consistent with an initial A2Π ← X2Π (ΔΩ=0) excitation in the parent cation, but the best-fit description of the fragment angular distributions requires use of higher terms in the Legendre expansion than is traditionally the case for a one-photon dissociation process. The additional angular modulation is rationalised by recognizing that the REMPI preparation laser induces unwanted (but unavoidable) parent ion fragmentation, leaving a spatially aligned distribution of ions for the intended photolysis study. Interpretation of the measured product branching ratios and recoil anisotropies is guided by complementary ab initio calculations of spin–orbit free potential energy curves for many of the lower lying electronic states of BrCl+ and subsequent semi-empirical inclusion of spin–orbit effects. The adiabatic potentials so derived exhibit a plethora of avoided crossings. Preliminary wavepacket propagations following excitation to the A2Π state show rapid flux redistribution to adiabatic potentials that correlate with the lower dissociation asymptotes, but are unable to reproduce the details of the experimentally determined product branching ratios.

3-Nitro-2-naphthalenemethanol: a photocleavable protecting group for carboxylic acids

Photocleavable protecting groups are important in synthesis and caging. Among many such groups, 2-nitrobenzyl and related groups have been found useful in many applications. However, most of the known 2-nitrobenzyl-based caging chromophores show either low quantum yield or the photolysis wavelength is not suitable for various applications. In this paper, we report 2-nitro-3-naphthalenemethanol (NNM) as an efficient photocleavable protecting group for molecules containing a carboxylic function. NNM possesses photochemical properties better than the 2-nitrobenzyl chromophores as it is photoactivatable at 380 nm in aqueous medium (CH 3CN/H 2O, 3:2 v/v) showing the desired photochemistry. The carboxylic acids are efficiently photoreleased from NNM-based esters in almost quantitative yield.

Heavy hydrides: H2Te ultraviolet photochemistry

The room-temperature ultraviolet absorption spectrum of H2Te has been recorded. Unlike other group-6 hydrides, it displays a long-wavelength tail that extends to 400 nm. Dissociation dynamics have been examined at photolysis wavelengths of 266 nm (which lies in the main absorption feature) and 355 nm (which lies in the long-wavelength tail) by using high-n Rydberg time-of-flight spectroscopy to obtain center-of-mass translational energy distributions for the channels that yield H atoms. Photodissociation at 355 nm yields TeH(2Pi1/2) selectively relative to the TeH(2Pi3/2) ground state. This is attributed to the role of the 3A' state, which has a shallow well at large R(H-TeH) and correlates to H+TeH(2Pi1/2). Note that the 2Pi1/2 state is analogous to the 2P1/2 spin-orbit excited state of atomic iodine, which is isoelectronic with TeH. The 3A' state is crossed at large R only by 2A", with which it does not interact. The character of 3A' at large R is influenced by a strong spin-orbit interaction in the TeH product. Namely, 2Pi1/2 has a higher degree of spherical symmetry than does 2Pi3/2 (recall that I(2P1/2) is spherically symmetric), and consequently 2Pi1/2 is not inclined to form either strongly bonding or antibonding orbitals with the H atom. The 3A'<--X transition dipole moment dominates in the long-wavelength region and increases with R. Structure observed in the absorption spectrum in the 380-400 nm region is attributed to vibrations on 3A'. The main absorption feature that is peaked at approximately 240 nm might arise from several excited surfaces. On the basis of the high degree of laboratory system spatial anisotropy of the fragments from 266 nm photolysis, as well as high-level theoretical studies, the main contribution is believed to be due to the 4A" surface. The 4A"<--X transition dipole moment dominates in the Franck-Condon region, and its polarization is in accord with the experimental observations. An extensive secondary photolysis (i.e., of nascent TeH) is observed at 266 and 355 nm, and the corresponding spectral features are assigned. Analyses of the c.m. translational energy distributions yield bond dissociation energies D0. For H2Te and TeH, these are 65.0+/-0.1 and 63.8+/-0.4 kcalmol, respectively, in good agreement with predictions that use high-level relativistic theory.

Synthesis of 1α-Hydroxyvitamin D5 Using a Modified Two Wavelength Photolysis for Vitamin D Formation

[reaction: see text] 1Alpha-hydroxyvitamin D5 (1) is a promising chemopreventive agent for breast cancer and is being developed as a drug. We report a synthesis for this vitamin D analogue which uses a photochemical method for the B-ring opening, leading to the conjugated triene system. The precursor 7-dehydrositosteryl acetate (4) obtained through a one-pot, five-step procedure, was completely free of the 4,6-diene isomer that usually forms in the 5,7-diene synthesis. The pre-vitamin isomer (11) was generated using a modified two-wavelength photolysis procedure that increases the yield for this step more than 3-fold compared to classically used photolysis. The 1alpha-hydroxylation step was performed on the 3-triethylsilyl-trans-vitamin D5 (17) obtained via the sulfur dioxide adduct of cis-vitamin D5, in an overall yield of 48%. Photoisomerization and deprotection completed the synthesis.

Ultraviolet photodissociation dynamics of the SH radical

Ultraviolet (UV) photodissociation dynamics of jet-cooled SH radical (in X 2pi(3/2), nu"=0-2) is studied in the photolysis wavelength region of 216-232 nm using high-n Rydberg atom time-of-flight technique. In this wavelength region, anisotropy beta parameter of the H-atom product is approximately -1, and spin-orbit branching fractions of the S(3P(J)) product are close to S(3P2):S(3P1):S(3P0)=0.51:0.36:0.13. The UV photolysis of SH is via a direct dissociation and is initiated on the repulsive 2sigma- potential-energy curve in the Franck-Condon region after the perpendicular transition 2sigma(-)-X 2pi. The S(3P(J)) product fine-structure state distribution approaches that in the sudden limit dissociation on the single repulsive 2sigma- state, but it is also affected by the nonadiabatic couplings among the repulsive 4sigma-, 2sigma-, and 4pi states, which redistribute the photodissociation flux from the initially excited 2sigma- state to the 4sigma- and 4pi states. The bond dissociation energy D0(S-H)=29,245+/-25 cm(-1) is obtained.

Photodissociation of state selected BrCl cations: Branching ratios and angular anisotropies of the Br product forming channels

High resolution velocity map ion imaging methods have been used to explore the photofragmentation dynamics of state-selected BrCl+(X2ΠΩ, v=0) cations following excitation in the wavelength range 370-420 nm. The parent ions are formed by 2 + 1 resonance enhanced multiphoton ionization (REMPI) via the [2Π1/2]5sσ, v=0 Rydberg level. Experimental measurables include the wavelength dependent branching ratios and recoil anisotropies of the various Br+(3PJ) + Cl(2PJ) product channels. Ground state Br+(3P2) fragments dominate the fragment ion yield throughout the photolysis wavelength range investigated, with spin-orbit excited Cl(2P1/2) atoms the preferred co-fragment whenever energetically allowed. All fragmentation channels show a predominantly parallel recoil anisotropy, consistent with an initial A2Π←X2Π (ΔΩ=0) excitation in the parent cation, but the best-fit description of the fragment angular distributions requires use of higher terms in the Legendre expansion than is traditionally the case for a one-photon dissociation process. The additional angular modulation is rationalised by recognizing that the REMPI preparation laser induces unwanted (but unavoidable) parent ion fragmentation, leaving a spatially aligned distribution of ions for the intended photolysis study. Interpretation of the measured product branching ratios and recoil anisotropies is guided by complementary ab initio calculations of spin-orbit free potential energy curves for many of the lower lying electronic states of BrCl+ and subsequent semi-empirical inclusion of spin-orbit effects. The adiabatic potentials so derived exhibit a plethora of avoided crossings. Preliminary wavepacket propagations following excitation to the A2Π state show rapid flux redistribution to adiabatic potentials that correlate with the lower dissociation asymptotes, but are unable to reproduce the details of the experimentally determined product branching ratios.

The 193 nm photolysis of NO2: NO(ν) vibrational distribution, O(1D) quantum yield and emission from vibrationally excited NO2

Time resolved Fourier transform infrared emission has been used to study the photolysis of NO2 and its dimer N2O4 at 193 nm. NO(ν) populations from the photolysis of NO2 show a bimodal distribution, peaking at ν = 5 and with a subsidiary maximum at ν = 14, close to the energetically allowed limit. The results are discussed in terms of previous measurements near this photolysis wavelength of the kinetic energies of the two possible O atomic fragments, O(3P) and O(1D), and on the electronic states of the parent molecule which can be populated. The O(1D) yield has been measured as 0.51 ± 0.04, in good agreement with previously reported values. Emission from vibrationally excited NO2 arises from the dissociation of N2O4 and is similar to that observed from photolysis at 248 nm.

Photolysis wavelength dependence of the translational anisotropy and the angular momentum polarization of O2(aΔg1) formed from the UV photodissociation of O3

The translational anisotropy and angular momentum polarization of the O(2)(a (1)Delta(g),v = 0;J = 15-27) molecular photofragment produced from the UV photodissociation of O(3) in the range from 270 to 300 nm have been determined using resonance-enhanced multiphoton ionization in conjunction with time-of-flight mass spectrometry. At the shortest photolysis wavelengths used, the fragments exhibit the anisotropic vector correlations expected from a prompt dissociation via the (1)B(2) <--(1)A(1) transition. Deviations from this behavior are observed at longer photolysis wavelengths with, in particular, the angular momentum orientation showing a significant reduction in magnitude. This indicates that the dissociation can no longer be described by a purely impulsive model and a change in geometry of the dissociating molecule is implied. This observation is substantiated by the variation of the translational anisotropy with photolysis wavelength. We also observe that the bipolar moments describing the angular momentum polarization of the odd J states probed are consistently lower in magnitude than those of the even J states and that this variation is observed for all photolysis wavelengths.

Velocity-map imaging study of the photodissociation of acetaldehyde

Velocity-map imaging studies are reported for the photodissociation of acetaldehyde over a range of photolysis wavelengths (317.5-282.5 nm). Images are obtained for both the HCO and CH3 fragments. The mean rotational energy of both fragments increases with photodissociation energy, with a lesser degree of excitation in the CH3 fragment. The CH3 images demonstrate that the CH3 fragments are rotationally aligned with respect to the recoil direction and this is interpreted, and well modeled, on the basis of a propensity for forming CH3 fragments with M approximately K, where M is the projection of the rotational angular momentum along the recoil direction. The origin of the CH3 rotation is conserved motion from the torsional and methyl-rocking modes of the parent molecule. Nonstatistical vibrational distributions for the CH3 fragment are obtained at higher energies.

Accurate Determination of the Absolute Quantum Yield for O(1D) Formation in the Photolysis of Ozone at 308 nm.

For Abstract see ChemInform Abstract in Full Text.

Accurate Determination of the Absolute Quantum Yield for O(1D) Formation in the Photolysis of Ozone at 308 nm

Absolute O('D) quantum yield from the photodissociation reaction of O 3 at 308 nm should be determined accurately with high precision for its atmospheric significance. This is because it has been used as a reference in previous relative measurements to obtain the O( 1 D) quantum yields as a function of photolysis wavelength in the near-UV region. In this study, the near-UV pulsed-laser photolysis of O 3 with the direct detection of the O( 1 D) and O( 3 P) photofragments using a technique of vacuum UV laser-induced fluorescence has been applied to determine the absolute O('D) quantum yield from O 3 photolysis. The absolute O( 1 D) quantum yield at 308.0 nm at room temperature (298 ′ 2 K) is determined to be 0.804 ′ 0.048 (95% confidence interval), in which the uncertainty is much smaller than the values recommended for use in atmospheric studies. The O( 1 D) quantum yield values at photolysis wavelengths between 307 and 311.5 nm are also presented.

Wavelength-Dependent Photolysis of n-Hexanal and n-Heptanal in the 280−330-nm Region

We have studied the photolysis of n-hexanal (CH3(CH2)4CHO) and n-heptanal (CH3(CH2)5CHO) at 5-nm intervals in the 280−330-nm region by using dye-laser photolysis combined with cavity ring-down spectroscopy. Their absorption cross sections have been obtained at each wavelength studied. The HCO radical is a photodissociation product of both aldehydes. The HCO radical quantum yields have been determined as a function of photolysis wavelength (λ), aldehyde pressure, and nitrogen buffer gas pressure. The HCO radical yields decrease with increasing aldehyde pressure (0.5−8 Torr for n-hexanal and 0.5−4 Torr for n-heptanal) because of the increasing HCO + HCO, HCO + R, and HCO + RCHO reactions (R = n-C5H11 for n-hexanal and n-C6H13 for n-heptanal) at higher aldehyde pressures and because of quenching by ground-state aldehydes. After separating the contribution of HCO radical reactions, the aldehyde-pressure quenching effect was still observed at all wavelengths. The HCO quantum yields and the ratios of quenching to unimolecular decay rate constants of excited aldehydes are given. The HCO quantum yields from n-hexanal photolysis are 0.12 ± 0.01, 0.15 ± 0.02, 0.14 ± 0.02, and 0.10 ± 0.01 at 305, 310, 315, and 320 nm, respectively, where the uncertainty (1σ) represents experimental scatter. The corresponding HCO quantum yields from n-heptanal photolysis are 0.14 ± 0.04, 0.15 ± 0.06, 0.10 ± 0.01, and 0.11 ± 0.02, respectively. A comparison of the HCO radical yields from the wavelength-dependent photolysis of n-hexanal and n-heptanal with that from n-pentanal photolysis indicates that the HCO radical yields from aldehyde photolysis do not vary with chain length for aldehydes with chain lengths that are longer than or equal to five carbon atoms. The dependence of the HCO quantum yield on nitrogen buffer gas pressure was examined between 6 and 387 Torr; no dependence was observed. The end products from 308-nm excimer-laser photolysis of both aldehydes were measured by mass spectrometry and FTIR. Evidence has been obtained for the occurrence of the Norrish II channel, and the photocyclization channels for both aldehydes and their yields have been obtained.