Photofragment Translational Spectroscopy Research Articles

Observation of ultrafast NH3 (Ã) state relaxation dynamics using a combination of time-resolved photoelectron spectroscopy and photoproduct detection

The ultrafast excited state relaxation of ammonia is investigated by resonantly exciting specific vibrational modes of the electronically excited NH(3) (Ã) state using three complementary femtosecond (fs) pump-probe techniques: time-resolved photoelectron, ion-yield and photofragment translational spectroscopy. Ammonia can be seen as a prototypical system for studying non-adiabatic dynamics and therefore offers a benchmark species for demonstrating the advantages of combining the aforementioned techniques to probe excited state dynamics, whilst simultaneously illuminating new aspects of ammonia's photochemistry. Time-resolved photoelectron spectroscopy (TRPES) provides direct spectroscopic evidence of σ* mediated relaxation of the NH(3) (Ã) state which manifests itself as coupling of the umbrella (ν(2)) and symmetric N-H stretch (ν(1)) modes in the photoelectron spectra. Time-resolved ion yield (TRIY) and time-resolved photofragment translation spectroscopy (TRPTS) grant a measure of the dissociation dynamics through analysis of the H and NH(2) photodissociation co-fragments. Initial vibrational level dependent TRIY measurements reveal photoproduct formation times of between 190 and 230 fs. Measurement of H-atom photoproduct kinetic energies enables investigation into the competition between adiabatic and non-adiabatic dissociation channels at the NH(3) (Ã)/NH(3) (X̃) conical intersection and has shown that upon non-adiabatic dissociation into NH(2) (X̃) + H, the NH(2) (X[combining tilde]) fragment is predominantly generated with significant fractions of internal vibrational energy.

Vibrational energy redistribution in catechol during ultraviolet photolysis

This article reports the striking interplay between the molecular structure and the photodissociation dynamics of catechol (a key dihydroxybenzene), identified using a combination of electronic spectroscopy, hydrogen (Rydberg) atom photofragment translational spectroscopy, density functional theory and second order approximate coupled cluster methods. We describe how the non-planar (C(1) symmetry) ← planar (C(s) symmetry) geometry change during S(1) (1(1)ππ*) ←S(0) excitation in catechol, as well as the presence of internal hydrogen bonding, can perturb the photodissociation dynamics relative to that of phenol (a monohydroxybenzene), particularly with respect to O-H bond fission via the lowest dissociative (1)πσ* state. For λ(phot) > 270 nm, O-H bond fission (of the non hydrogen bonded hydroxyl moiety) is deduced to proceed via H atom tunnelling from the photo-prepared 1(1)ππ* state into the lowest (1)πσ* state of the molecule. The vibrational energy distribution in the resulting catechoxyl product changes notably as λ(phot) is tuned on resonance with either the v' = 0, m(2)' = 1(+) or m(2)' = 2(+) torsional levels of the photo-prepared 1(1)ππ* state: the product state distribution is highly sensitive to the degree of OH torsional excitation (m(2)) prepared during photo-excitation. It is deduced that such torsional excitation can be redistributed very efficiently into ring puckering (and likely also in-plane ring stretch) vibrations as the molecule tunnels to its repulsive 1(1)πσ* state and dissociates. These observations can be rationalised by consideration of the photo-prepared nuclear wavefunctions. Analysis of the product vibrational energy distribution also reveals that the O-H bond strength of the non hydrogen bonded O-H moiety in catechol, D(0)(H-catechoxyl) ≤ 27 480 ± 50 cm(-1), ∼2500 cm(-1) lower than that of the sole O-H bond in bare phenol. As a consequence, the vertical excitation energy of the 1(1)πσ* state in catechol is reduced relative to that in phenol, yielding a particularly broad distribution of product vibrations for λ(phot) < 270 nm. This study highlights the interplay between molecular geometry and redistribution of vibrational energy during ultraviolet photolysis of phenols.

Ultrafast dynamics of aniline following 269–238 nm excitation and the role of the S2(π3s/πσ*) state

Femtosecond time-resolved photoelectron imaging is employed to investigate ultrafast electronic relaxation in aniline, a prototypical aromatic amine. The molecule is excited at wavelengths between 269 and 238 nm. We observe that the S2(pi3s/pi sigma*) state is populated directly during the excitation process at all wavelengths and that the population bifurcates to two decay pathways. One of these involves ultrafast relaxation from the Rydberg component of S2(pi3s/pi sigma*) to the S1(Pi Pi)* state, from which it relaxes back to the electronic ground state on a much longer timescale. The other appears to involve motion along the pi sigma* dissociative potential energy surface. At higher excitation energies, the dominant excitation is to the S3(pi pi*) state, which undergoes extremely efficient electronic relaxation back to the ground state. Our study supports some conclusions reached from H-atom photofragment translational spectroscopy measurements and pump-probe photoionization measurements and contradicts some others.

Contrasting the excited state reaction pathways of phenol and para-methylthiophenol in the gas and liquid phases

To explore how the solvent influences primary aspects of bond breaking, the gas and solution phase photochemistries of phenol and ofpara-methylthiophenol are directly compared using, respectively, H (Rydberg) atom photofragment translation spectroscopy and femtosecond transient absorption spectroscopy. Approaches are demonstrated that allow explicit comparisons of the nascent product energy disposals and dissociation mechanisms in the two phases. It is found, at least for the case of the weakly perturbing cyclohexane environment, that most aspects of the primary reaction dynamics of the isolated molecule are reproduced in solution. Specifically, in the gas phase, both molecules can undergo fast X-H (X = O, S) bond dissociation upon excitation with short wavelengths (193 < lambda(pump) < 216 nm), following population of the dissociative S2 (1 1(pi sigma*)) state. Product electronic branching, vibrational and translational energy disposals are determined. Photolysis of phenol and para-methylthiophenol in solution at 200 nm results in formation of vibrationally excited radicals on a timescale shorter than 200 fs. Excitation of para-methylthiophenol at 267 nm reaches close to the S1 (1 1(pipi*))/S2 (11(pi sigma*)) conical intersection (CI): ultrafast dissociation is observed in both the isolated and solution systems-again indicating direct dissociation on the S2 potential energy surface. Comparing results for this precursor at different excitation energies, the extent of geminate recombination and the derived H-atom ejection lengths in the condensed phase photolyses are in qualitative agreement with the translational energy release measured in the gas phase studies. Conversely, excitation of phenol at 267 nm prepares the system in its S1 state at an energy well below its S1/S2 CI; the slow O-H bond fission inferred in the gas phase experiments is observed directly in the time-resolved studies in cyclohexane solution via the appearance of phenoxyl radical absorption after -1 ns, with only S1 excited state absorption discernible at earlier delay times. The slow O-H bond fission in solution provides additional evidence for a tunnelling dissociation mechanism, where the H atom tunnels beneath the lower diabats of the S2/S1 CI. Finally, the photodissociation of phenol clusters in solution is considered, where evidence is presented that the O-H dissociation coordinate is impeded in H-bonded dimers.

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).

Evidence for Synchronous Concerted Three-Body Dissociation of Propenal to C2H2+CO+H2

Synchronous dissociation: The transition structure of propenal leading to the fragments C2H2+CO+H2 was identified using the computational method B3LYP/6-311G(d,p). Arrows indicate the linear momenta of the three fragments (see picture). The synchronous concerted three-body dissociation process was observed by photofragment translational spectroscopy.

Vibrationally Resolved Photofragment Translational Spectroscopy of CH3I from 277 to 304 nm with Increasing Effect of the Hot Band

The photodissociation dynamics of CH(3)I from 277 to 304 nm is studied with our mini-TOF photofragment translational spectrometer. A single laser beam is used for both photodissociation of CH(3)I and REMPI detection of iodine. Many resolved peaks in each photofragment translational spectrum reveal the vibrational states of the CH(3) fragment. There are some extra peaks showing the existence of the hot-band states of CH(3)I. After careful simulation with consideration of the hot-band effect, the distribution of vibrational states of the CH(3) fragment is determined. The fraction σ of photofragments produced from the hot-band CH(3)I varies from 0.07 at 277.38 nm to 0.40 at 304.02 nm in the I* channel and from 0.05 at 277.87 nm to 0.16 at 304.67 nm in the I channel . E(int)/E(avl) of photofragments from ground-state CH(3)I remains at about 0.03 in the I* channel for all four wavelengths, but E(int)/E(avl) decreases from 0.09 at 277.87 nm to 0.06 at 304.67 nm in the I channel . From the ground-state CH(3)I, the quantum yield Φ(I*) is determined to be 0.59 at 277 nm and 0.05 at 304 nm. The curve-crossing probability P(cc) from the hot-band CH(3)I is lower than that from the ground-state CH(3)I. The potential energy at the curve-crossing point is determined to be 32,740 cm(-1).

Photodissociation dynamics of ClOOCl at 248.4 and 308.4 nm

The dynamics of ClOOCl photodissociation at 248.4 and 308.4 nm was studied with photofragment translational spectroscopy. At 248.4 nm photoexcitation, the observed products are Cl, O(2), ClO and O. Product translational energy distributions P(E) and anisotropy parameters β were deduced from the measured time-of-flight spectra of the Cl, O(2), and ClO photoproducts. The photodissociation mechanisms have been discussed and compared with available theoretical results. Synchronous and fast sequential breaking of the two Cl-O bonds may both contribute to the dissociation. The relative product yields for [ClO]: [Cl] was measured to be 0.15 ± 0.04:1. The relative amounts of [O]:[O(2)] products were estimated to be 0.12:1. The branching ratios among the Cl + O(2) + Cl:ClO + ClO:ClO + Cl + O product channels were estimated to be 0.82:0.08:0.10. At 308.4 nm excitation, time-of-flight spectra of the O(2) and ClO photoproducts were recorded while there was interference from Cl(2) impurity in detecting the Cl product. Nonetheless, the observed ClO yield relative to the O(2) yield at 308.4 nm is 1.5 times that at 248.4 nm. The branching ratio between the Cl + O(2) + Cl:ClO + ClO product channels was estimated to be 0.81:0.19 at 308.4 nm. This result suggests that the ClO product may contribute a noticeable yield in the photolysis of ClOOCl at the atmospherically important wavelengths above 300 nm.

A complete look at the multi-channel dissociation of propenal photoexcited at 193 nm: branching ratios and distributions of kinetic energy

We observed fifteen photofragments upon photolysis of propenal (acrolein, CH(2)CHCHO) at 193 nm using photofragment translational spectroscopy and selective vacuum-ultraviolet (VUV) photoionization. All the photoproducts arise from nine primary and two secondary dissociation pathways. We measured distributions of kinetic energy of products and determined branching ratios of dissociation channels. Dissociation to CH(2)CHCO + H and CH(2)CH + HCO are two major primary channels with equivalent branching ratios of 33%. The CH(2)CHCO fragment spontaneously decomposes to CH(2)CH + CO. A proportion of primary products CH(2)CH from the fission of bond C-C of propenal further decompose to CHCH + H but secondary dissociation HCO → H + CO is negligibly small. Binary dissociation to CH(2)CH(2) (or CH(3)CH) + CO and concerted three-body dissociation to C(2)H(2) + CO + H(2) have equivalent branching ratios of 14%-15%. The other channels have individual branching ratios of ∼1%. The production of HCCO + CH(3) indicates the formation of intermediate methyl ketene (CH(3)CHCO) and the production of CH(2)CCH + OH and CH(2)CC + H(2)O indicate the formation of intermediate hydroxyl propadiene (CH(2)CCHOH) from isomerization of propenal. Distributions of kinetic energy release and dissociation mechanisms are discussed. This work provides a complete look and profound insight into the multi-channel dissociation mechanisms of propenal. The combination of a molecular beam apparatus and synchrotron VUV ionization allowed us to untangle the complex mechanisms of nine primary and two secondary dissociation channels.

Photodissociation dynamics of the tert-butyl radical via photofragment translational spectroscopy at 248 nm

The photodissociation dynamics of the tert-butyl radical (t-C(4)H(9)) were investigated using photofragment translational spectroscopy. The tert-butyl radical was produced from flash pyrolysis of azo-tert-butane and dissociated at 248 nm. Two distinct channels of approximately equal importance were identified: dissociation to H + 2-methylpropene, and CH(3) + dimethylcarbene. Neither the translational energy distributions that describe these two channels nor the product branching ratio are consistent with statistical dissociation on the ground state, and instead favor a mechanism taking place on excited state surfaces.

Position matters: competing O–H and N–H photodissociation pathways in hydroxy- and methoxy-substituted indoles

H (Rydberg) atom photofragment translational spectroscopy (HRA-PTS) and complete active space with second order perturbation theory (CASPT2) methods have been used to explore the competing N-H and O-H bond dissociation pathways of 4- and 5-hydroxyindoles (HI) and methoxyindoles (MI). When 4-HI was excited to bound (1)L(b) levels, (λ(phot) ≤ 284.893 nm) O-H bond fission was demonstrated by assignment of the structure within the resulting total kinetic energy release (TKER) spectra. By analogy with phenol, dissociation was deduced to occur by H atom tunnelling under the barrier associated with the lower diabats of the (1)L(b)/(1)πσ*((OH)) conical intersection (CI). No evidence was found for a significant N-H bond dissociation yield at these or shorter excitation wavelengths (284.893 ≥ λ(phot) ≥ 193.3 nm). Companion studies of 4-MI revealed different reaction dynamics. In this case, N-H bond fission is deduced to occur at λ(phot) ≤ 271.104 nm, by direct excitation to the (1)πσ*((NH)) state. Analysis of the measured TKER spectra implies a mechanism wherein, as in pyrrole, the (1)πσ*((NH)) state gains oscillator strength by intensity borrowing from nearby bound states with higher oscillator strengths. HRA-PTS studies of 5-HI, in contrast, showed no evidence for O-H bond dissociation when excited on (1)L(b) levels. The present CASPT2 calculations assist in rationalizing this observation: the area underneath the (1)L(b)/(1)πσ* CI diabats in 5-HI is ~60% greater than the corresponding area in 4-HI and O-H bond dissociation by tunnelling is thus much less probable. Only by reducing the wavelength to ≤ 255 nm were signs of N-H and/or O-H bond dissociation identified. By comparison with companion 5-MI studies, we deduce little O-H bond fission in 5-HI at λ(phot) > 235 nm and that N-H bond fission is the dominant source of H atoms in the wavelength region 255 > λ(phot) > 235 nm. The very different dissociation dynamics of 4- and 5-HI are traced to the position of the -OH substituent, and its effect on the overall electronic structure.

Linking photochemistry in the gas and solution phase: S–H bond fission in p-methylthiophenol following UV photoexcitation

Gas-phase H (Rydberg) atom photofragment translational spectroscopy and solution-phase femtosecond-pump dispersed-probe transient absorption techniques are applied to explore the excited state dynamics of p-methylthiophenol connecting the short time reactive dynamics in the two phases. The molecule is excited at a range of UV wavelengths from 286 to 193 nm. The experiments clearly demonstrate that photoexcitation results in S-H bond fission--both in the gas phase and in ethanol solution-and that the resulting p-methythiophenoxyl radical fragments are formed with significant vibrational excitation. In the gas phase, the recoil anisotropy of the H atom and the vibrational energy disposal in the p-MePhS radical products formed at the longer excitation wavelengths reveal the operation of two excited state dissociation mechanisms. The prompt excited state dissociation motif appears to map into the condensed phase also. In both phases, radicals are produced in both their ground and first excited electronic states; characteristic signatures for both sets of radical products are already apparent in the condensed phase studies after 50 fs. No evidence is seen for either solute ionisation or proton coupled electron transfer--two alternate mechanisms that have been proposed for similar heteroaromatics in solution. Therefore, at least for prompt S-H bond fissions, the direct observation of the dissociation process in solution confirms that the gas phase photofragmentation studies indeed provide important insights into the early time dynamics that transfer to the condensed phase.

Theoretical Investigation of 1,2-Interchange of a Chlorine Atom and Methyl Group in 1,1-Dichloroacetone

A recent photofragment translational spectroscopy study of 1,1-dichloroacetone at 193 nm reported two primary unimolecular decomposition channels: C-Cl bond cleavage and elimination of HCl in a 9:1 ratio, respectively. The HCl translational energy distribution was bimodal suggesting two distinct decomposition pathways that were assumed to be 1,1-HCl loss forming a carbene and a 1,3-HCl elimination reaction forming a biradical ( Butler , L. J. ; Liu , Y. ; Lau , K. ; McCunn , L. R. ; Fitzpatrick , B. L. ; Bell , J. M. ; Krisch , M. J. J. Phys. Chem. A 2007 , 111 , 5968. ). An alternative two-step mechanism for HCl loss has been proposed involving interchange of a chlorine atom and a CH(3) group converting 1,1-dichloroacetone into 2-chloropropanoyl chloride followed by either a 1,2-HCl or 2,3-HCl elimination reaction. This alternative mechanism was computationally explored with density functional theory using B3PW91/6-31G(d',p') and unimolecular rate constants were calculated. The theoretical rate constant ratio for loss of HCl and the mean HCl translation energy for each elimination channel were in excellent agreement with the experimental results.

The ultraviolet photodissociation of axial and equatorial conformers of 3-pyrroline

Resolved sets of photoproducts arising from the photodissociation of axial and equatorial conformers of 3-pyrroline have been observed using H(Rydberg) atom photofragment translational spectroscopy following excitation in the wavelength range of 250-213 nm. 3-pyrroline (alternatively 2,5-dihydropyrrole) is a five membered partially saturated heterocycle in which the bonding around the N atom is pyramidal (sp(3) hybridized) and the N-H bond can lie either axial or equatorial to the ring. Careful analysis of total kinetic energy release data derived from H atom time-of-flight measurements reveals excitation of the 3-pyrrolinyl cofragment consistent with N-H bond fission in both the axial and equatorial conformers. This allows determination of the energy difference between the ground state conformers to be 340±50 cm(-1) and the N-H bond strength for axial and equatorial conformers as 31,610±50 and 31,270±50 cm(-1), respectively.

Photodissociation dynamics of the phenyl radical via photofragment translational spectroscopy

Photofragment translational spectroscopy was used to study the photodissociation dynamics of the phenyl radical C(6)H(5) at 248 and 193 nm. At 248 nm, the only dissociation products observed were from H atom loss, attributed primarily to H+o-C(6)H(4) (ortho-benzyne). The observed translational energy distribution was consistent with statistical decay on the ground state surface. At 193 nm, dissociation to H+C(6)H(4) and C(4)H(3)+C(2)H(2) was observed. The C(6)H(4) fragment can be either o-C(6)H(4) or l-C(6)H(4) resulting from decyclization of the phenyl ring. The C(4)H(3)+C(2)H(2) products dominate over the two H loss channels. Attempts to reproduce the observed branching ratio by assuming ground state dynamics were unsuccessful. However, these calculations assumed that the C(4)H(3) fragment was n-C(4)H(3), and better agreement would be expected if the lower energy i-C(4)H(3)+C(2)H(2) channel were included.

Dynamical insights into π1σ∗ state mediated photodissociation of aniline

This article reports a comprehensive study of the mechanisms of H atom loss in aniline (C(6)H(5)NH(2)) following ultraviolet excitation, using H (Rydberg) atom photofragment translational spectroscopy. N-H bond fission via the low lying (1)pi sigma(*) electronic state of aniline is experimentally demonstrated. The (1)pi sigma(*) potential energy surface (PES) of this prototypical aromatic amine is essentially repulsive along the N-H stretch coordinate, but possesses a shallow potential well in the vertical Franck-Condon region, supporting quasibound vibrational levels. Photoexcitation at wavelengths (lambda(phot)) in the range 293.859 nm > or = lambda(phot) > or = 193.3 nm yields H atom loss via a range of mechanisms. With lambda(phot) resonant with the 1(1)pi pi(*) <-- S(0) origin (293.859 nm), H atom loss proceeds via, predominantly, multiphoton excitation processes, resonantly enhanced at the one photon energy by the first (1)pi pi(*) excited state (the 1(1)pi pi(*) state). Direct excitation to the first few quasibound vibrational levels of the (1)pi sigma(*) state (at wavelengths in the range 269.513 nm > or = lambda(phot) > or = 260 nm) induces N-H bond fission via H atom tunneling through an exit barrier into the repulsive region of the (1)pi sigma(*) PES, forming anilino (C(6)H(5)NH) radical products in their ground electronic state, and with very limited vibrational excitation; the photo-prepared vibrational mode in the (1)pi sigma(*) state generally evolves adiabatically into the corresponding mode of the anilino radical upon dissociation. However, as the excitation wavelength is reduced (lambda(phot) < 260 nm), N-H bond fission yields fragments with substantially greater vibrational excitation, rationalized in terms of direct excitation to 1(1)pi pi(*) levels, followed by coupling to the (1)pi sigma(*) PES via a 1(1)pi pi(*)/(1)pi sigma(*) conical intersection. Changes in product kinetic energy disposal once lambda(phot) approaches approximately 230 nm likely indicate that the photodissociation pathways of aniline proceed via direct excitation to the (higher) 2(1)pi pi(*) state. Analysis of the anilino fragment vibrational energy disposal-and thus the concomitant dynamics of (1)pi sigma(*) state mediated photodissociation-provides a particularly interesting study of competing sigma(*) <-- pi and pi(*) <-- pi absorption processes and develops our appreciation of the photochemistry of aromatic amines. It also allows revealing comparisons with simple amines (such as ammonia and methylamine) as well as the isoelectronic species, phenol. This study yields a value for the N-H bond strength in aniline, D(0)(H-anilino) = 31630+/-40 cm(-1).

Product rotational Franck–Condon oscillations in HOD (Jka , kc ) dissociation

The technique of H(D) atom photofragment translation spectroscopy has been used to investigate the dissociation of jet-cooled HOD molecules following excitation to individual rovibrational levels of its (1B1) Rydberg state near 124 nm. Spectra have been recorded for both the D + OH and H + OD dissociation channels. The branching ratios between OH/OD(X , high v, low N), OH/OD(X , low v, high N), and OH/OD(A ) channels vary between different parent rotational levels, and between the OH and OD products. The variation between the OH and OD channels can be attributed to an isotopic mass effect on the rotational axes which influences non-adiabatic coupling strengths. In addition, there is a population alternation with product rotational quantum number N for the OH/OD(X, high v, low N) product, most striking for the OH case, the sense of which is correlated with the value of the parent pseudo-quantum number . It is shown that this is a consequence of recoil forces which lead to a closing of the HOD angle to near 90° accompanying non-adiabatic transfer from the state to the (1B1) state. The oscillation in population then derives from the symmetry properties around this angle of the product rotational wavefunctions. This suggests that the symmetry-induced population oscillation can only occur for a narrowly defined angular dissociation path. Model calculations also explain the different structure of the spin-orbit doubled low N band heads for each v(OH/OD) which also alternates for even and odd . The necessary conditions to observe these phenomena are discussed.

Exploring the mechanisms of H atom loss in simple azoles: Ultraviolet photolysis of pyrazole and triazole

The photophysics of gas phase pyrazole (C(3)N(2)H(4)) and 2H-1,2,3-triazole (C(2)N(3)H(3)) molecules following excitation at wavelengths in the range 230 nm>or=lambda(phot)>or=193.3 nm has been investigated using the experimental technique of H (Rydberg) atom photofragment translational spectroscopy. The findings are compared with previous studies of pyrrole (C(4)N(1)H(5)) and imidazole (C(3)N(2)H(4)), providing a guide to H atom loss dynamics in simple N-containing heterocycles. CASPT2 theoretical methods have been employed to validate these findings. Photoexcitation of pyrazole at the longest wavelengths studied is deduced to involve pi( *)<--pi excitation, but photolysis at lambda(phot)</=214 nm is characterized by rapid N-H bond fission on a (1)pisigma( *) potential energy surface. The eventual pyrazolyl radical products are formed in a range of vibrational levels associated with both the ground ((2)A(2)) and first excited ((2)B(1)) electronic states as a result of nonadiabatic coupling at large N-H bond lengths. The excitation energy of the lowest (1)pisigma( *) state of pyrazole is found to be significantly higher in energy than that of pyrrole and imidazole. Similar studies of 2H-1,2,3-triazole reveal that the lowest (1)pisigma( *) state is yet higher in energy and not accessible following excitation at lambda(phot)>or=193.3 nm. The N-H bond strength of pyrazole is determined as 37 680+/-40 cm(-1), significantly greater than that of the N-H bonds in pyrrole and imidazole. The correlation between the photochemistry of azoles and the number and position of nitrogen atoms within the ring framework is discussed in terms of molecular symmetry and orbital electron density. A photodissociation channel yielding H atoms with low kinetic energies is also clearly evident in both pyrazole and 2H-1,2,3-triazole. Companion studies of pyrazole-d(1) suggest that these slow H atoms arise primarily from the N-H site, following pi( *)<--pi excitation, and subsequent internal conversion and/or unintended multiphoton absorption processes.

Dynamics of multi-channel dissociation of tetrahydrofuran photoexcited at 193 nm: distributions of kinetic energy, angular anisotropies and branching ratios

We investigated the photodissociation dynamics of tetrahydrofuran (c-C(4)H(8)O) at 193.3 nm in a molecular-beam apparatus using photofragment-translational spectroscopy and direct vacuum-ultraviolet (VUV) photoionization. Five dissociation channels leading to products with m/z ratios appropriate for CH(2)CH(2)CH(2) + H(2)CO, CH(2)CHCH(2) + CH(2)OH, H + CH(2)CH(2) + CH(2)CHO, CH(2)CH(2) + CH(3) + HCO and CH(2)CH(2) + CH(2)CO + H(2) were identified; their branching ratios were determined to be 0.40, 0.25, 0.04 0.29 and 0.02, respectively. Secondary dissociations from nascent products CH(2)CH(2)CH(2)CHO to CH(2)CH(2) + CH(2)CHO and from CH(2)CH(2)O to CH(3) + HCO and likely to CH(2)CO + H(2) were observed. We measured distributions of product kinetic energy, average kinetic-energy release, and fractions in translation for each dissociation channel. The formation of CH(2)CHCH(2) + CH(2)OH indicates that hydrogen migration occurs before complete fragmentation. All photofragments have nearly isotropic angular distributions, with |beta| values less than 0.05. The photodissociation of tetrahydrofuran into five channels is proposed to proceed mainly on the ground state potential-energy surface following ring opening and efficient internal conversions.