Photodissociation of uracil

We investigate the photochemistry and photodissociation dynamics of uracil by two-colour photofragment Doppler spectroscopy and by two-colour slice imaging at excitation wavelengths between 268 and 235 nm. We observe the loss of a hydrogen atom upon excitation into the pipi* state. The angular distribution indicates a statistical process, while the translational energy distribution agrees with a dissociation that takes place on the electronic ground state. The pipi* state most likely deactivates via the lower-lying npi* state. In addition there is evidence for a second pathway: direct decay of the pipi* state to the electronic ground state with subsequent dissociation. Experiments on uracil-1,3-D(2) show that there is no site selectivity in the dissociation process. No evidence was found for the direct dissociation via a pisigma* excited state that seems to be relevant in the photochemistry of adenine and many other heterocyclic molecules. Overall, the photochemistry of uracil is similar to that of thymine.     

Photofragment translational spectroscopy of ICl near 304 and 280 nm: Observation of an intense hot band effect

The photodissociation dynamics of ICl has been studied near 304 and 280 nm on a simple miniature time of flight (mini-TOF) photofragment translational spectrometer with a short pulse of a weak acceleration field. An intense hot band effect was observed. Many small peaks were resolved in each photofragment translational spectrum (PTS). Based on simulations, the principal peaks were assigned not only to the different photodissociation channels (1) I + Cl, (2) I + Cl*, (3) I* + Cl, or (4) I* + Cl*, but also to the different chlorine isotopes (35Cl and Cl). Moreover, some extra peaks showed the existence of an intense hot 37 band effect from vibrationally excited ICl molecules, though only a few percent of ICl molecules remained in the vibrationally excited states in our supersonic molecular beam. Based on the spectra near 304 nm, the quantum yield Ф of each channel, the curve crossing, and the branching fraction σ from each transition state were determined.     

Photodissociation dynamics of allyl bromide at 234, 265, and 267 nm

The photodissociation dynamics of allyl bromide was investigated at 234, 265, and 267 nm. A two-dimensional photofragment ion velocity imaging technique coupled with a [2+1] resonance-enhanced multiphoton ionization scheme was utilized to obtain the angular and translational energy distributions of the nascent Br* (2P1/2) and Br (2P3/2) atoms. The Br fragments show a bimodal translational energy distribution, while the Br* fragments reveal one translational energy distribution. The vertical excited energies and the mixed electronic character of excited states were calculated at ab initio configuration interaction method. It is presumed that the high kinetic energy bromine atoms are attributed to the predissociation from 1(pipi*) or 1(pisigma*) state to the repulsive 1(nsigma*) state, and to the direct dissociation from 3(nsigma*) and 3(pisigma*) states, while the low kinetic energy bromine atoms stem from internal conversion from the lowest 3(pipi*) state to 3(pisigma*) state.     

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) (?) 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) (?) 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) (?)/NH(3) (X?) conical intersection and has shown that upon non-adiabatic dissociation into NH(2) (X?) + H, the NH(2) (X[combining tilde]) fragment is predominantly generated with significant fractions of internal vibrational energy.     

The photodissociation dynamics of tetrachloroethylene

We present a direct current slice imaging study of tetrachloroethylene (C(2)Cl(4)) photodissociation, probing the resulting ground state Cl ((2)P(3/2)) and spin-orbit excited state Cl* ((2)P(1/2)) products. We report photofragment images, total translational energy distributions and the product branching ratio of Cl*/Cl following dissociation at 235 and 202 nm, obtained using a two-color reduced-Doppler dissociation/probe. Near 235 nm, the Cl translational energy distribution shows a peak at the limit of the available energy, indicating a direct dissociation through a σ*(C-Cl) ← π (C=C) transition, which is superimposed on a broader underlying distribution. The ground state Cl image and associated translational energy distribution at 202 nm is broad and peaked at lower energy, suggesting either internal conversion to the ground state or a lower excited state prior to dissociation. The Cl* images are similarly broad at both wavelengths. The branching ratio is presented as a function of recoil energy, but after integration shows a near-statistical average of Cl:Cl* as 70:30 at both wavelengths. All the images are largely isotropic, with anisotropy parameters (β) of 0.05 ± 0.03.     

Internal state distribution of the CF fragment from the 193 nm photodissociation of CFCl and CFBr

The dynamics of the 193 nm photodissociation of the CFCl and CFBr molecules have been investigated in a molecular beam experiment. The CFCl and CFBr parent molecules were generated by pyrolysis of CHFCl2 and CFBr3, respectively, and the CFCl and the CF photofragment were detected by laser fluorescence excitation. The 193 nm attenuation cross section of CFCl was determined from the reduction of the CF photofragment signal as a function of the photolysis laser fluence. The internal state distribution was derived from the analysis of laser fluorescence excitation spectra in the A 2Sigma+-X 2Pi band system. A very low degree of rotational excitation, with essentially equal A' and A" Lambda-doublet populations, and no vibrational excitation were found in the CF photofragment. The energy available to the photofragments is hence predominantly released as translational energy. The CF internal state distribution is consistent with the dissociation of a linear intermediate state. Considerations of CFCl electronic states suggest that a bent Rydberg state is initially excited.     

Dynamics of the 193 nm photodissociation of dichlorocarbene

The dynamics of the 193 nm photodissociation of the CCl2 molecule have been investigated in a molecular beam experiment. The CCl2 parent molecule was generated in a molecular beam by pyrolysis of CHCl3, and both CCl2 and the CCl photofragment were detected by laser fluorescence excitation. The 193 nm attenuation cross section was estimated from the reduction of the CCl2 signal as a function of the photolysis laser fluence. The internal state distribution of the CCl photofragment was derived from analysis of laser fluorescence excitation spectra in the A 2Delta-X 2Pi band system. Most of the energy available to the CCl(X 2Pi)+Cl fragments appears as translational energy. The CCl fragment rotational energy is much less than predicted in an impulsive model. The excited electronic state appears to dissociate indirectly, through coupling with a repulsive state arising from the ground-state CCl(X 2Pi)+Cl asymptote. The identity of the initially excited electronic state is discussed on the basis of what is known about the CCl2 electronic states.     

Statistical vs. non-statistical deactivation pathways in the UV photo-fragmentation of protonated tryptophan-leucine dipeptide

The excited state dynamics of protonated tryptophan-leucine ions WLH+, generated in an electrospray source, is investigated by photo-induced fragmentation in the gas phase, using femtosecond laser pulses. Two main features arise from the experiment. Firstly, the initially excited pipi* state decays very quickly with 2 time constants of 1 and 10 ps. Secondly, the transient signals recorded on different fragments are not the same which indicates two competing primary fragmentation processes. One involves a direct dissociation from the excited state that gives evidence for a non-statistical deactivation path. The other is attributed to a statistical decay following internal conversion to the ground electronic surface.     

Photoinduced processes in protonated tryptamine

The electronic excited state dynamics of protonated tryptamine ions generated by an electrospray source have been studied by means of photoinduced dissociation technique on the femtosecond time scale. The result is that the initially excited state decays very quickly within 250 fs. The photoinduced dissociation channels observed can be sorted in two groups of fragments coming from two competing primary processes on the singlet electronic surface. The first one corresponds to a hydrogen-atom loss channel that creates a tryptamine radical cation. The radical cation subsequently fragments to smaller ions. The second process is internal conversion due to the H-atom recombination on the electronic ground state. Time-dependent density functional theory calculations show that an excited pisigma* state dissociative along the protonated amino N-H stretch crosses both the locally excited pipi* state and the electronic ground state S(0) and thus triggers the photofragmentation reactions. The two processes have equivalent quantum yields, approximately equal to 50% of the fragments coming from the H-atom loss reaction. The two primary reaction paths can clearly be distinguished by their femtosecond pump/probe dynamics recorded on the different fragmentation channels.     

Photodissociation dynamics of the chromophores of the amino acid tyrosine: p-methylphenol, p-ethylphenol, and p-(2-aminoethyl)phenol

The photodissociation of p-methylphenol, p-ethylphenol, and p-(2-aminoethyl)phenol, chromophores of the amino acid tyrosine, was studied separately for each compound in a molecular beam at 248 nm using multimass ion imaging techniques. They show interesting side-chain size-dependent dissociation properties. Only one dissociation channel, that is, H atom elimination, was observed for both p-methylphenol and p-ethylphenol. The photofragment translational energy distributions and potential energy surfaces from ab initio calculation suggest that H atom elimination occurs from a repulsive excited state. On the other hand, the H atom elimination channel is quenched completely by internal conversion and/or intersystem crossing in p-(2-aminoethyl)phenol. Only C-C bond cleavage was observed from p-(2-aminoethyl)phenol. The photofragment translational energy distribution shows a slow component and a fast component. The fast component results from dissociation on an electronic excited state, but the slow component occurs only after the internal conversion to the ground electronic state. Comparison with the photodissociation of phenol and ethylbenzene is made.     

Observation of the A1A" state of isocyanogen

The A1A" state of isocyanogen, CNCN, is observed using photofragment fluorescence excitation spectroscopy in a room temperature cell and in a molecular beam. The spectra are highly congested, but progressions that correspond to the Franck-Condon active C-N-C bending vibration in the excited state are evident. Linewidth measurements indicate that the excited state lifetime is <10 ps. These measurements are consistent with previous ab initio calculations, which predicted a bent excited state with a short lifetime due to predissociation. Although we do not believe that we have observed the origin band of the electronic transition, we place an upper limit of 42,523 cm(-1) on the energy of the excited state zero point level.     

Photochemistry of aryl halides: Photodissociation dynamics

In recent years, the photodissociation dynamics of aryl halides has been a subject of intensive studies, which is closely related to the atmospheric chemistry. Here we present a review on the photochemistry of aryl halides, with emphasis on the recent progress in photodissociation dynamics at 266 nm by using photofragment translational spectroscopy. The ab initio calculations have also been employed to investigate those photodissociation processes. It has been found that the photodissociation of aryl halides at 266 nm is attributed to the nonadiabatic process via intersystem crossings from bound singlet excited state to triplet excited state and/or via internal conversion from bound singlet excited state to ground state. Also, the substitution effects in the photodissociation dynamics of aryl halides are discussed.     

Molecular photofragment orientation in the photodissociation of H2O2 at 193 nm and 248 nm

Angular momentum orientation has been observed in the OH(X(2)Π, v = 0) fragments generated by circularly polarized photodissociation of H(2)O(2) at 193 nm and 248 nm. The magnitude and sign of the orientation are strongly dependent on the OH(X) photofragment rotational state. In addition to conventional laser induced fluorescence methods, Zeeman quantum beat spectroscopy has also been used as a complementary tool to probe the angular momentum orientation parameters. The measured orientation at 193 nm is attributed solely to photodissociation via the ?(1)A state, even though at this wavelength H(2)O(2) is excited near equally to both the ?(1)A and B(1)B electronic states. This observation is confirmed by measurements of the photofragment orientation at 248 nm, where access to the ?(1)A state dominates. Several possible mechanisms are discussed to explain the observed photofragment orientation, and a simple physical model is developed, which includes the effects of the polarization of the parent molecular rotation upon absorption of circularly polarized light. Good agreement between the experimental and simulation results is obtained, lending support to the validity of the model. It is proposed that photofragment orientation arises mainly from the coupling of the parent rotational angular momentum with that induced during photofragmentation.     

High resolution photofragment translational spectroscopy of the near UV photolysis of indole: dissociation via the 1pi sigma* state

The fragmentation dynamics of indole molecules following excitation at 193.3 nm, and at a number of different wavelengths in the range 240 < or = lambda(phot) < or = 286 nm, have been investigated by H Rydberg atom photofragment translational spectroscopy. The longer wavelength measurements have been complemented by measurements of excitation spectra for forming parent and fragment ions by two (or more) photon ionisation processes. Analysis identifies at least three distinct contributions to the observed H atom yield, two of which are attributable to dissociation of indole following radiationless transfer from the 1pi pi* excited states (traditionally labelled 1L(b) and 1L(a)) prepared by UV single photon absorption. The structured channel evident in total kinetic energy release (TKER) spectra recorded at lambda(phot) < or = 263 nm is rationalised in terms of N-H bond fission following initial pi* <-- pi excitation and subsequent coupling to the 1pi sigma* potential energy surface via a conical intersection between the respective surfaces--thereby validating recent theoretical predictions regarding the importance of this process (Sobolewski et al., Phys. Chem. Chem. Phys., 2002, 4, 1093). Analysis provides an upper limit for the N-H bond strength in indole: D0(H-indolyl) < or = 31,900 cm(-1). Unimolecular decay of highly vibrationally excited ground state molecules formed by internal conversion from the initially prepared 1pi pi* states is a source of (slow) H atoms but their contribution to the TKER spectra measured in the present work is dwarfed by that from H atoms generated by one or more (unintended but unavoidable) multiphoton processes.     

The decay from the dark npi* excited state in uracil: an integrated CASPT2/CASSCF and PCM/TD-DFT study in the gas phase and in water

By integrating the results of MS-CASPT2/CASSCF and TD-PBE0 calculations, we propose a mechanism for the decay of the excited dark state in pyrimidine, fully consistent with all the available experimental results. An effective conical intersection (CI-npi) exists between the spectroscopic pi/pi* excited state (Spi) and a dark n/pi* state (Sn), and a fraction of the population decays to the minimum of Sn (Sn-min). The conical intersection between Sn and the ground-state is not involved in the decay mechanism, because of its high energy gap with respect to Sn-min. On the other hand, especially in hydrogen bonding solvents, the energy gap between Sn-min and CI-npi is rather small. After thermalization in Sn-min, the system can thus recross CI-npi and then quickly proceed on the Spi barrierless path toward the conical intersection with the ground state.     

State-to-state reaction dynamics of CH3I photodissociation at 304 nm

The detailed reaction dynamics of CH(3)I photodissociation at 304 nm were studied by using high-resolution long time-delayed core-sampling photofragment translation spectroscopy. The vibrational state distributions of the photofragment, i.e., CH(3), are directly resolved due to the high kinetic resolution of this experiment for the first time. CH(3) radicals produced from I((3)Q(0+)), I((1)Q(1) <--( 3)Q(0+)), and I((3)Q(1)) channels are populated in different vibrational state distributions. The I((3)Q(0+)) and I((3)Q(1)) channels show only progressions in the nu2'(a2") umbrella bending mode, and the I((1)Q(1) <-- (3)Q(0+)) channel shows both progression in the nu2' umbrella bending mode and a small amount of excitation in the nu1'(a1') C-H stretching mode. The photodissociation processes from the vibrational hot band of CH(3)I (upsilon3 = 1, upsilon3 = 2) were also detected, primarily because of the absorption probability from the vibrational excited states, i.e., hot bands are relatively enhanced. Photofragments from the hot bands of CH(3)I show a cold vibrational distribution compared to that from the vibrational ground state of CH(3)I. The I* quantum yield and the curve crossing possibility were also studied for the ground vibrational state of CH(3)I. The potential energy at the curve crossing point was calculated to be 32 790 cm(-1) by using the one-dimensional Landau-Zener model.     

CO2 laser excitation of triethylsilane: Time resolved luminescence of diethylsilyl radical

The infrared multiphoton excitation of triethylsilane in the gas phase, with a pulsed CO₂ laser at high intensities (I > 700 MW/cm²), produced an intense luminescence. The spectrum and time profile of this luminescence was studied as a function of pressure, and laser frequency. The radiative lifetime of this emission was 357 ± 10 ns, and the quenching rates by Cl₂ and NO were determined from lifetime measurements. A reasonable mechanism for the interpretation of this luminescence involves the initial infrared multiphoton decomposition of triethylsilane, followed by the secondary infrared multiphoton excitation of the primary photofragment diethylsilyl radical, which subsequently undergoes relaxation to an excited electronic state. The addition of O₂ resulted in a new chemiluminescence at shorter wavelengths, which corresponds to the SiO* chromophore group. © 1994 John Wiley & Sons, Inc.     

Near-ultraviolet photodissociation of thiophenol

H(D) Rydberg atom photofragment translational spectroscopy has been used to investigate the dynamics of H(D) atom loss C6H5SH(C6H5SD) following excitation at many wavelengths lambda phot in the range of 225-290 nm. The C6H5S cofragments are formed in both their ground (X(2)B1) and first excited ((2)B2) electronic states, in a distribution of vibrational levels that spreads and shifts to higher internal energies as lambda(phot) is reduced. Excitation at lambda(phot) > 275 nm populates levels of the first (1)pi pi* state, which decay by tunnelling to the dissociative (1)pi sigma* state potential energy surface (PES). S-H torsional motion is identified as a coupling mode facilitating population transfer at the conical intersection (CI) between the diabatic (1)pi pi* and (1)pi sigma* PESs. At shorter lambda(phot), the (1)pi sigma* state is deduced to be populated either directly or by efficient vibronic coupling from higher (1)pipi* states. Flux evolving on the (1)pi sigma* PES samples a second CI, at longer R(S-H), between the diabatic (1)pi sigma* and ground ((1)pi pi) PESs, where the electronic branching between ground and excited state C6H5S fragments is determined. The C6H5S(X(2)B1) and C6H5S((2)B2) products are deduced to be formed in levels with, respectively, a' and a' vibrational symmetry-behavior that reflects both Franck-Condon effects (both in the initial photoexcitation step and in the subsequent in-plane forces acting during dissociation) and the effects of the out-of-plane coupling mode(s), nu11 and nu16a, at the (1)pi sigma*/(1)pi pi CI. The vibrational state assignments enabled by the high-energy resolution of the present data allow new and improved estimations of the bond dissociation energies, D0(C6H5S-H) < or = 28,030 +/- 100 cm(-1) and D0(C6H5S-D) < or = 28,610 +/- 100 cm(-1), and of the energy separation between the X(2)B1 and (2)B2 states of the C6H5S radical, T(00) = 2800 +/- 40 cm(-1). Similarities, and differences, between the measured energy disposals accompanying UV photoinduced X-H (X = S, O) bond fission in thiophenol and phenol are discussed.     

Ab initio study on deactivation pathways of excited 9H-guanine

The complete active space with second-order perturbation theory/complete active space self-consistent-field method was used to explore the nonradiative decay mechanism for excited 9H-guanine. On the 1pipi* (1L(a)) surface we determined a conical intersection (CI), labeled (S0pipi*)(CI), between the 1pipi* (1L(a)) excited state and the ground state, and a minimum, labeled (pipi*)min. For the 1pipi* (1L(a)) state, its probable deactivation path is to undergo a spontaneous relaxation to (pipi*)min first and then decay to the ground state through (S0pipi*)(CI), during which a small activation energy is required. On the 1n(N)pi* surface a CI between the 1n(N)pi* and 1pipi* (1L(a)) states was located, which suggests that the 1n(N)pi* excited state could transform to the 1pipi* (1L(a)) excited state first and then follow the deactivation path of the 1pipi* (1L(a)) state. This CI was also possibly involved in the nonradiative decay path of the second lowest 1pipi* (1L(b)) state. On the 1n(O)pi* surface a minimum was determined. The deactivation of the 1n(O)pi* state to the ground state was estimated to be energetically unfavorable. On the 1pisigma* surface, the dissociation of the N-H bond of the six-membered ring is difficult to occur due to a significant barrier.     

Photofragment translational spectroscopy at 304 nm from C-H symmetric stretch excited CH3I [v 1 = 1]

The 304 nm photodissociation of the C-H symmetric stretch excited CH3I[v1=1,v2=0](v1 denotes the C-H symmetric stretch mode,and v2 denotes the umbrella mode)is studied with our simple photofragment translational spectrometer.An IR laser is used to excite the ground state CH3I[0,0]to the C-H symmetric stretch excited CH3I[1,0].With IR laser OFF and ON,the fractions of photofragments CH3(ν1,ν2)from the 304 nm photodissociation of CH3I[1,0]have been determined through the photofragment translational spectra(PTS)from measuring I and I*and also through the PTS from measuring CH3(0,0)(1,0)(0,1)and(1,1).The experimental results show that the C-H symmetric stretch vibration(v1=1)in parent molecules is about 66%retained in the photofragments in the I channel,but only 24%in the I*channel.The populations of photofragments CH3(0,2)and(0,3)are higher than CH3(0,0)and(0,1),showing strong inverted population both in I and I*channels.