High-n Rydberg Atom Time-of-flight Research Articles

Ultraviolet Photodissociation Dynamics of the 1-Methylallyl Radical.

The ultraviolet (UV) photodissociation dynamics of the 1-methylallyl (1-MA) radical were studied using the high-n Rydberg atom time-of-flight (HRTOF) technique in the wavelength region of 226-244 nm. The 1-MA radicals were produced by 193 nm photodissociation of the 3-chloro-1-butene and 1-chloro-2-butene precursor. The 1 + 1 REMPI spectrum of 1-MA agrees with the previous UV absorption spectrum in this wavelength region. Quantum chemistry calculations show that the UV absorption is mainly attributed to the 3pz Rydberg state (perpendicular to the allyl plane). The H atom photofragment yield (PFY) spectrum of 1-MA from 3-chloro-1-butene displays a broad peak around 230 nm, while that from 1-chloro-2-butene peaks at ∼236 nm. The translational energy distributions of the H atom loss product channel, P (ET)'s, show a bimodal distribution indicating two dissociation pathways in 1-MA. The major pathway is isotropic in product angular distribution with β ∼ 0 and has a low fraction of average translational energy in the total excess energy, ⟨fT⟩, in the range of 0.13-0.17; this pathway corresponds to unimolecular dissociation of 1-MA after internal conversion to form 1,3-butadiene + H. The minor pathway is anisotropic with β ∼ -0.23 and has a large ⟨fT⟩ of ∼0.62-0.72. This fast pathway suggests a direct dissociation of the methyl H atom on a repulsive excited state surface or the repulsive part of the ground state surface to form 1,3-butadiene + H. The fast/slow pathway branching ratio is in the range of 0.03-0.08.

Ultraviolet photochemistry of the 2-buten-2-yl radical.

The ultraviolet (UV) photodissociation dynamics of the 2-buten-2-yl (C4H7) radical were studied using the high-n Rydberg atom time-of-flight (HRTOF) technique in the photolysis region of 226-246 nm. 2-Buten-2-yl radicals were generated by 193 nm photodissociation of the precursor 2-chloro-2-butene. The H-atom photofragment yield (PFY) spectrum of 2-buten-2-yl is broad, peaking at 234 nm. Quantum chemistry calculations show that the UV absorption is due to the 3py and 3px Rydberg states (parallel to the plane of CC double bond). The translational energy distributions of the H-atom loss product channel, P(ET)'s, of 2-buten-2-yl show a bimodal distribution indicating two dissociation pathways. The major pathway peaks at ET ∼ 7 kcal mol-1 with a nearly constant fraction of average ET in the total excess energy, 〈fT〉, at ∼0.11-0.12. This main pathway has an isotropic product angular distribution with β ∼ 0, consistent with the unimolecular dissociation of a hot 2-buten-2-yl radical following internal conversion from the electronically excited state, resulting in the formation of 2-butyne + H (∼84%) and 1,2-butadiene + H (∼16%). Additionally, there is a minor non-statistical pathway with an isotropic angular distribution. The minor pathway peaks at ET ∼ 35 kcal mol-1 in the P(ET) distributions and exhibits a large 〈fT〉 of ∼0.40-0.46. This fast pathway suggests a direct dissociation of the methyl H-atom on a repulsive excited state surface or on the repulsive part of the ground state surface, forming 1,2-butadiene + H. The fast/slow pathway branching ratio is in the range of 0.03-0.08.

Ultraviolet photodissociation of 2-methylallyl radical

Ultraviolet photodissociation dynamics of 2-methylallyl radical from the 3p Rydberg state were investigated in the wavelength region of 226–244 nm using the high-n Rydberg atom time-of-flight (HRTOF) technique. The 2-methylallyl radicals were generated by 193 nm photolysis of 3-chloro-2-methyl-1-propene precursors. The photofragment yield spectrum of H-atom products increases in intensity with decreasing wavelengths in 226–244 nm. The TOF spectra of H-atom products show a bimodal structure. The predominant product channel (with ∼98% branching ratio) has a kinetic energy release peaking at ∼7 kcal/mol, with an average ratio of ET in the total available energy, (fT), of ∼0.18 in 226–244 nm and an isotropic product angular distribution. At the low ET, isotropic component is from statistical unimolecular decomposition of highly vibrationally excited hot 2-methylallyl to the methylenecyclopropane+H products, following internal conversion from the excited electronic state. The minor product channel (with ∼2% branching ratio) has a large kinetic energy peaking at ∼50 kcal/mol, with (fT)≈0.63 and an anisotropic angular distribution (β≈−0.2). At the high ET, anisotropic component is non-statistical and is postulated to be from direct loss of H atom via the 3p Rydberg state or repulsive part of the ground state to the 1,3-butadiene+H products.

Two-photon dissociation dynamics of the mercapto radical.

Two-photon dissociation dynamics of the SH/SD radicals are investigated using the high-n Rydberg atom time-of-flight (HRTOF) technique. The H/D(2S) + S(1D) and H/D(2S) + S(1S) products are observed in the dissociation of the SH/SD radicals on the 22Π and B2Σ+ repulsive states, from sequential two-photon excitation via the A2Σ+ (v' = 0, J' = 0.5-2.5) state. The angular distributions of both H/D(2S) + S(1D) and H/D(2S) + S(1S) product channels are anisotropic. The anisotropy parameter (β) of the H(2S) + S(1D) products is ∼-0.8 ± 0.1 (-0.9 ± 0.05 for SD), and that of the H(2S) + S(1S) products is ∼1.3 ± 0.3 (1.2 for SD). The anisotropic angular distributions indicate that the SH/SD radicals promptly dissociate on the repulsive 22Π and B2Σ+ potential energy curves (PECs) along with some non-adiabatic crossings, leading to the H/D(2S) + S(1D) and H/D(2S) + S(1S) products, respectively. The bond dissociation energy of the ground-state X2Π3/2 SH/SD to the ground-state H/D(2S) + S(3P2) products is measured to be D0(S-H) = 29 253 ± 20 cm-1 and D0(S-D) = 29 650 ± 20 cm-1, respectively. The dissociation energy of the A2Σ+ state SH/SD to the H/D(2S) + S(1D) products is derived to be D0[S-H(A)] = 7659 ± 20 cm-1 and D0[S-D(A)] = 7940 ± 20 cm-1.

Ultraviolet Photodissociation Dynamics of the Cyclohexyl Radical: The H-Atom Product Channel.

The ultraviolet (UV) photodissociation dynamics of the jet-cooled cyclohexyl (c-C6H11) radical is studied using the high-n Rydberg atom time-of-flight (HRTOF) technique. The cyclohexyl radical is produced by the 193 nm photodissociation of chlorocyclohexane and bromocyclohexane and is examined in the photolysis wavelength region of 232-262 nm. The H-atom photofragment yield (PFY) spectrum contains a broad peak centered at 250 nm, which is in good agreement with the UV absorption spectrum of the cyclohexyl radical and assigned to the 3p Rydberg states. The translational energy distributions of the H-atom loss product channel, P(ET)'s, are bimodal, with a slow (low ET) component peaking at ∼6 to 7 kcal/mol and a fast (high ET) component peaking at ∼44-48 kcal/mol. The fraction of the average translational energy in the total excess energy, ⟨fT⟩, is in the range of 0.16-0.25 in the photolysis wavelength region of 232-262 nm. The H-atom product angular distribution of the slow component is isotropic, while that of the fast component is anisotropic with an anisotropy parameter of β ≈ 0.5-0.7. The bimodal product translational energy and angular distributions indicate two dissociation pathways to the H + C6H10 products in cyclohexyl. The high-ET anisotropic component is from a repulsive, prompt dissociation on a repulsive potential energy surface coupling with the Rydberg excited states to produce H + cyclohexene. The low-ET isotropic component is consistent with the unimolecular dissociation of hot radical on the ground electronic state after internal conversion from the Rydberg states. The similarity of the photodissociation dynamics of the cyclohexyl radical to the previously studied small linear and branched alkyls expands on the understanding of the dissociation dynamics of alkyl radicals to include larger cyclic alkyl radicals.

State-to-state predissociation dynamics of hydroxyl radical via the A2Σ+ state

ABSTRACT Photo-predissociation dynamics of jet-cooled hydroxyl radical (OH) via several rovibrational levels (v′ = 2–4, N′ = 0–2, J′ = 0.5–2.5, F 1) in the A2Σ+ state are studied using the high–n Rydberg-atom time-of-flight (HRTOF) technique. Spin–orbit branching fractions and angular distributions of the H(2S) + O(3PJ = 2,1,0) product channels are measured in the product translational energy distributions. The A2Σ+ v′ = 2 and 3 states of OH predissociate predominantly via the single 14Σ− repulsive state, while the A2Σ+ v′ = 4 state predissociates via the 14Σ−, 12Σ−, and 14Π repulsive states and quantum interferences among these dissociation pathways play an important role in the O(3PJ = 2,1,0) product fine-structure state distribution. The experimental O(3PJ) product spin–orbit branching ratios are in excellent agreement with the full quantum multichannel scattering calculations that include all the interactions in the crossing region, recoupling zone, and asymptotic zone [Parlant and Yarkony, J. Chem. Phys. 110, 363 (1999)]. The product angular distributions strongly depend on the rotational transitions and rotational levels in the A2Σ+ state. The measured anisotropy parameters are in reasonable agreement with predictions from the simulation programme (Betaofnu) [Kim et al., J. Chem. Phys. 125, 133316 (2006)], using dissociation lifetime, excitation frequency, rotational level, and rotational constant. The predissociation time scales of OH A2Σ+ are determined to be 130 ± 30 ns and ≥ 14 ± 3 ps for the v′ = 2, N′ = 2, J′ = 2.5 and v′ = 4, N′ = 0, J′ = 0.5 levels, respectively.

Two-photon dissociation dynamics of hydroxyl radical

Two-photon dissociation dynamics of the OH radical is studied using the high-n Rydberg atom time-of-flight (HRTOF) technique. The H(2S)+O(1D) and H(2S)+O(1S) product channels are observed in the dissociation of the OH radical on the 22Π and B2Σ+ repulsive states, respectively, from sequential two-photon excitation via the A2Σ+ (v′=2, J′=0.5−2.5) state. Both H+O product channels have anisotropic angular distributions, with β=−0.97 for H(2S)+O(1D) and 1.97 for H(2S)+O(1S). The anisotropic angular distributions are consistent with a mechanism of OH direct dissociation on the repulsive potential energy curves (PECs) leading to the H+O products. The OH bond dissociation energy D0(O−H) is determined to be 35580±15 cm−1.

Ultraviolet photodissociation dynamics of 1-pentyl radical

The ultraviolet (UV) photodissociation of jet-cooled 1-pentyl radical is investigated in the wavelength region of 236−254 nm using the high-n Rydberg-atom time-of-flight (HRTOF) technique. The H-atom photofragment yield spectrum of the 1-pentyl radical shows a broad UV absorption feature peaking near 245 nm, similar to the 2pz →3s absorption bands of ethyl and n-propyl. The center-of-mass translational energy distribution, P (ET), of the H+C5H10 product channel is bimodal, with a slow peak at ~5 kcal/mol and a fast peak at ~50 kcal/mol. The fraction of the average translational energy release in the total available energy, ⟨fT⟩, is 0.30, with those of the slow and fast components being 0.13 and 0.58, respectively. The slow component has an isotropic product angular distribution, while the fast component is anisotropic with an anisotropy parameter ~0.4. The bimodal translational energy and angular distributions of the H+C5H10 products indicate two H-atom elimination channels in the photodissociation of 1-pentyl: (i) a direct, prompt dissociation from the electronic excited state and/or the repulsive part of the ground electronic state potential energy surface; and (ii) a unimolecular dissociation of internally hot radical in the ground electronic state after internal conversion from the electronic excited state.

Ultraviolet Photodissociation Dynamics of the 1-Propenyl Radical.

Ultraviolet (UV) photodissociation dynamics of jet-cooled 1-propenyl radical (CHCHCH3) were investigated at the photolysis wavelengths from 224 to 248 nm using high-n Rydberg atom time-of-flight (HRTOF) technique. The 1-propenyl radicals were produced from 193 nm photolysis of 1-chloropropene and 1-bromopropene precursors. The photofragment yield (PFY) spectra of the H atom product have a broad peak centered at 230 nm. The H + C3H4 product translational energy P(ET) distribution's peak near ∼8 kcal/mol, and the fraction of average translational energy in the total available energy, ⟨fT⟩, is nearly a constant of ∼0.12 from 224 to 248 nm. The H atom product has an isotropic angular distribution with the anisotropy parameter β ≈ 0. Quasiclassical trajectory calculations were also carried out using an ab initio ground-state potential energy surface for dissociation of 1-propenyl at the excitation energy of 124 kcal/mol (230 nm). The calculated branching ratios are 60% to the methyl + acetylene products, 16% to H + propyne, 4% to H + allene, and 1% to H + cyclopropene. The experimental and calculated P(ET) distributions of the H + C3H4 products at 230 nm are in a qualitative agreement, suggesting that the H + propyne dissociation is the main H atom product channel. The calculated dissociation time scale on the ground electronic state is ∼1 ps, shorter than but close to the time scale of >10 ps for the overall UV photodissociation implied by the isotropic H atom product angular distribution. The UV photodissociation mechanism of 1-propenyl can be described as unimolecular decomposition of hot 1-propenyl radical on the ground electronic state following internal conversion from the electronically excited states of 1-propenyl.

Ultraviolet Photodissociation Dynamics of the Allyl Radical via the B̃(2)A1(3s), C̃(2)B2(3py), and Ẽ(2)B1(3px) Electronic Excited States.

Ultraviolet (UV) photodissociation dynamics of jet-cooled allyl radical via the B̃(2)A1(3s), C̃(2)B2(3py), and Ẽ(2)B1(3px) electronically excited states are studied at the photolysis wavelengths from 249 to 216 nm using high-n Rydberg atom time-of-flight (HRTOF) and resonance-enhanced multiphoton ionization (REMPI) techniques. The photofragment yield (PFY) spectra of the H atom products are measured using both allyl chloride and 1,5-hexadiene as precursors of the allyl radical and show a broad peak centered near 228 nm, whereas the previous UV absorption spectra of the allyl radical peak around 222 nm. This difference suggests that, in addition to the H + C3H4 product channel, another dissociation channel (likely CH3 + C2H2) becomes significant with increasing excitation energy. The product translational energy release of the H + C3H4 products is modest, with the P(ET) distributions peaking near 8.5 kcal/mol and the fraction of the average translational energy in the total excess energy, ⟨fT⟩, in the range 0.22-0.18 from 249 to 216 nm. The P(ET)'s are consistent with production of H + allene and H + propyne, as suggested by previous experimental and theoretical studies. The angular distributions of the H atom products are isotropic, with the anisotropy parameter β ≈ 0. The H atom dissociation rate constant from the pump-probe study gives a lower limit of 1 × 10(8)/s. The dissociation mechanism is consistent with unimolecular decomposition of the hot allyl radical on the ground electronic state after internal conversion of the electronically excited state.

H-atom Dissociation Channels in Ultraviolet Photochemistry of m-Pyridyl Radical

The H atom production channels in the ultraviolet (UV) photochemistry of m-pyridyl radical (m-C5H4N) were investigated at excitation wavelengths from 224 nm to 248 nm by high-n Rydberg atom time-of-flight (HRTOF) technique. The photofragment yield (PFY) spectrum of the H atoms indicates a broad UV absorption feature near 234 nm. The product kinetic energy release is small; the average product kinetic energy at the wavelengths from 224 nm to 248 nm is 0.12 to 0.19 of the maximum excess energy (assuming the lowest energy product channel, H+cyanovinylacetylene). The product kinetic energy distributions are consistent with the H+cyanovinylacetylene, H+3,4-pyridyne, and H+2,3-pyridyne product channels, with H+cyanovinylacetylene as the main H-loss channel. The angular distributions of the H-atom products are isotropic. After the UV electronic excitation, the m-pyridyl radical undergoes internal conversion to the ground electronic state and then unimolecular dissociation to the H+cyanovinylacetylene, H+3,4-pyridyne, and H+2,3-pyridyne products. The dissociation mechanism of the m-pyridyl radical is similar to that of the o-pyridyl radical reported in the early study.

Vacuum ultraviolet photodissociation dynamics of methanol at 121.6 nm

Vacuum ultraviolet (VUV) photodissociation of CH3OH and CH3OD at 121.6nm was studied using high-n Rydberg atom time-of-flight (HRTOF) technique. The H-atom product TOF spectrum of CH3OH and D-atom spectrum of CH3OD have bimodal distributions. O–H (CH3O+H(D)) and C–H (CH2OH(D)+H) dissociations contribute to the fast component. The slow component corresponds to CH2O+H+H(D) production, mainly from three-body photodissociation, plus small contributions from secondary dissociation of CH3O and CH2OH(D). The CH2O+H+H formation is the major photodissociation pathway, with a branching ratio of (CH2O+H+H):(CH3O+H):(CH2OH+H)≈1:0.25:0.15.

Ultraviolet Photodissociation Dynamics of the o-Pyridyl Radical

Ultraviolet (UV) photodissociation dynamics of jet-cooled o-pyridyl radical (o-C5H4N) was studied in the photolysis region 224-246 nm using the high-n Rydberg atom time-of-flight (HRTOF) technique. The o-pyridyl radicals were produced from 193 nm photolysis of 2-chloropyridine and 2-bromopyridine precursors. The H-atom photofragment yield (PFY) spectrum contains a broad peak in this wavelength region and reveals the UV absorption feature of o-pyridyl for the first time. The translational energy distributions of the H-atom loss product channel, P(ET)'s, peak at ∼7 kcal/mol, and the fraction of average translational energy in the total excess energy, <fT>, is nearly constant at ∼0.18 in the region 224-242 nm. The P(ET) distribution indicates the production of the lowest energy dissociation products, H + cyanovinylacetylene. The H-atom product angular distribution is isotropic. The dissociation mechanism is consistent with unimolecular dissociation of the hot o-pyridyl radical to H + cyanovinylacetylene after internal conversion from the electronically excited state.

Ultraviolet photodissociation dynamics of the benzyl radical

Ultraviolet (UV) photodissociation dynamics of jet-cooled benzyl radical via the 4(2)B(2) electronically excited state is studied in the photolysis wavelength region of 228 to 270 nm using high-n Rydberg atom time-of-flight (HRTOF) and resonance enhanced multiphoton ionization (REMPI) techniques. In this wavelength region, H-atom photofragment yield (PFY) spectra are obtained using ethylbenzene and benzyl chloride as the precursors of benzyl radical, and they have a broad peak centered around 254 nm and are in a good agreement with the previous UV absorption spectra of benzyl. The H + C(7)H(6) product translational energy distributions, P(E(T))s, are derived from the H-atom TOF spectra. The P(E(T)) distributions peak near 5.5 kcal mol(-1), and the fraction of average translational energy in the total excess energy, <f(T)>, is ∼0.3. The P(E(T))s indicate the production of fulvenallene + H, which was suggested by recent theoretical studies. The H-atom product angular distribution is isotropic, with the anisotropy parameter β ≈ 0. The H/D product ratios from isotope labeling studies using C(6)H(5)CD(2) and C(6)D(5)CH(2) are reasonably close to the statistical H/D ratios, suggesting that the H/D atoms are scrambled in the photodissociation of benzyl. The dissociation mechanism is consistent with internal conversion of the electronically excited benzyl followed by unimolecular decomposition of the hot benzyl radical on the ground state.

Ultraviolet Photodissociation Dynamics of the Propargyl Radical

Ultraviolet (UV) photodissociation dynamics of jet-cooled propargyl (C(3)H(3)) radical is studied in the photolysis wavelength region of 230 to 250 nm with high-n Rydberg atom time-of-flight (HRTOF) and resonance enhanced multiphoton ionization (REMPI) techniques. In this wavelength region, the photofragment yield (PFY) spectra of the H + C(3)H(2) product channel are obtained by using propargyl chloride, allene, and propyne as precursors of the C(3)H(3) radicals, and they have a broad peak centered near 240 nm and are in good agreement with the previous UV absorption spectrum of C(3)H(3) by Fahr et al. The H + C(3)H(2) product translational energy distributions, P(E(T))'s, are obtained from all three precursors and are essentially the same. The P(E(T)) distributions peak at approximately 5 kcal/mol, and the fraction of average translational energy in the total excess energy, f(T), is approximately 0.3. The H-atom product angular distribution is isotropic, with the anisotropy parameter beta approximately 0. The dissociation mechanism is consistent with internal conversion of the electronically excited propargyl followed by unimolecular decomposition on the ground state. Our study supports the previously observed UV absorption spectrum of propargyl near 240 nm by Fahr et al. and is in general agreement with the results in the UV photodissociation of propargyl by the groups of Chen and Neumark, but disagrees with the recent theoretical calculations by Eisfeld.

Ultraviolet photodissociation of the SD radical in vibrationally ground and excited states

Ultraviolet (UV) photodissociation dynamics of the SD radical in vibrationally ground and excited states (X (2)Pi(3/2), v'' = 0-5) are investigated in the photolysis wavelength region of 220 to 244 nm using the high-n Rydberg atom time-of-flight (HRTOF) technique. The UV photodissociation dynamics of SD (X (2)Pi(3/2)) from v'' = 0-5 are similar to each other and to that of SH studied previously. The anisotropy parameter of the D-atom product is approximately -1; the spin-orbit branching fractions of the S((3)P(J)) products are essentially constant, with an average S((3)P(2)) : S((3)P(1)) : S((3)P(0)) = 0.51 : 0.37 : 0.12. The UV photolysis of SD is a direct dissociation from the repulsive (2)Sigma(-) state following the perpendicular (2)Sigma(-)-X (2)Pi excitation. The S((3)P(J)) product fine-structure state distributions approach that in the sudden limit dissociation on the single repulsive (2)Sigma(-) curve, but they are also affected by nonadiabatic couplings among the repulsive (4)Sigma(-), (2)Sigma(-), and (4)Pi states. A bond dissociation energy D(0)(S-D) = 29 660 +/- 25 cm(-1) is obtained.

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.

H + NO2 Channels in the Photodissociation of HONO at 193.3 nm

The H + NO2 channels in the 193.3-nm photodissociation of jet-cooled HONO have been examined by using high-n Rydberg-atom time-of-flight (HRTOF) technique. Center-of-mass (CM) translational energy distribution and energy-dependent angular distribution of the photoproducts reveal that the NO2 fragments are produced in at least three electronic states: ground X̃2A1 and excited Ã2B2 and B̃2B1 (and/or C̃2A2) states. The overall average CM product translational energy is 〈ET〉 = 0.3Eavail. NO2 fragments are highly vibrationally excited in each of these electronic states, and in particular, a long vibrational progression of NO2 bending mode in the Ã2B2 state has been observed. Branching ratios of the NO2 electronic states are estimated: X̃2A1:Ã2B2:B̃2B1/C̃2A2 ≈ 0.13:0.21:0.66. The OH bond photodissociation of HONO from the second electronically excited singlet state Β1Α‘ proceeds via multiple dissociation pathways. The H + NO2(Ã2B2) product channel is via a direct dissociation (presumably in a near-planar fragmentation geometry) and has a large translational energy release, a specific NO2 bending vibration population (indicating a significant change of the ONO angle during dissociation), and an anisotropic product angular distribution (suggesting a short excited B̃1A‘ state lifetime with respect to dissociation). The H + NO2(X̃2A1) channel could be produced from a triplet excited state (which likely has a repulsive barrier along the O−H dissociation coordinate) following intersystem crossing or from the ground state of HONO after internal conversion. The H + NO2(B̃2B1) channel requires nonadiabatic processes in a planar geometry, while in nonplanar geometries, it can be directly produced from the HONO(B̃1A‘) state, consistent with its large branching ratio.