Ultraviolet photolysis of adenine: dissociation via the 1pi sigma* state

High resolution total kinetic energy release (TKER) spectra of the H atom fragments resulting from photodissociation of jet-cooled adenine molecules at 17 wavelengths in the range 280>lambda(phot)>214 nm are reported. TKER spectra obtained at lambda(phot)>233 nm display broad, isotropic profiles that peak at low TKER ( approximately 1800 cm(-1)) and are largely insensitive to the choice of excitation wavelength. The bulk of these products is attributed to unintended multiphoton dissociation processes. TKER spectra recorded at lambda(phot)相似文献   

High resolution photofragment translational spectroscopy studies of the near ultraviolet photolysis of phenol

The fragmentation dynamics of gas phase phenol molecules following excitation at many wavelengths in the range 279.145 > or = lambdaphot > or = 206.00 nm have been investigated by H Rydberg atom photofragment translational spectroscopy. Many of the total kinetic energy release (TKER) spectra so derived show structure, the analysis of which confirms the importance of O-H bond fission and reveals that the resulting phenoxyl cofragments are formed in a very limited subset of their available vibrational state density. Spectra recorded at lambdaphot > or = 248 nm show a feature centered at TKER approximately 6500 cm(-1). These H atom fragments, which show no recoil anisotropy, are rationalized in terms of initial S1<--S0 (pi*<--pi) excitation, and subsequent dissociation via two successive radiationless transitions: internal conversion to ground (S0) state levels carrying sufficient O-H stretch vibrational energy to allow efficient transfer towards, and passage around, the conical intersection (CI) between the S0 and S2(1pisigma*) potential energy surfaces (PESs) at larger R(O-H), en route to ground state phenoxyl products. The observed phenoxyl product vibrations indicate that parent modes nu16a and nu11 can both promote nonadiabatic coupling in the vicinity of the S0S2 CI. Spectra recorded at lambdaphot < or = 248 nm reveal a faster, anisotropic distribution of recoiling H atoms, centered at TKER approximately 12,000 cm(-1). These we attribute to H+phenoxyl products formed by direct coupling between the optically excited S1(1pi pi*) and repulsive S2(1pi sigma*) PESs. Parent mode nu16b is identified as the dominant coupling mode at the S1/S2 CI, and the resulting phenoxyl radical cofragments display a long progression in nu18b, the C-O in-plane wagging mode. Analysis of all structured TKER spectra yields D0(H-OC6H5) = 30,015 +/- 40 cm(-1). The present findings serve to emphasize two points of wider relevance in contemporary organic photochemistry: (i) The importance of 1) pi sigma* states in the fragmentation of gas phase heteroaromatic hydride molecules, even in cases where the 1pi sigma* state is optically dark. (ii) The probability of observing strikingly mode-specific product formation, even in "indirect" predissociations, if the fragmentation is driven by ultrafast nonadiabatic couplings via CIs between excited (and ground) state PESs.     

High resolution photofragment translational spectroscopy studies of the near ultraviolet photolysis of imidazole

The fragmentation dynamics of imidazole molecules following excitation at 193.3 nm and at many wavelengths in the range of 210< or =lambda(phot)< or =240 nm have been investigated by H Rydberg atom photofragment translational spectroscopy. Long wavelength excitation within this range results in population of the 1 (1)A(")((1)pisigma(*)) excited state, but the 2 (1)A(')<--X (1)A(')(pi(*)<--pi) transition becomes the dominant absorption once lambda(phot)< or =220 nm. The measured energy disposals show parallels with those found in recent studies of the UV photolysis of pyrrole [Cronin et al., Phys Chem. Chem. Phys. 6, 5031 (2004)]. The total kinetic energy release (TKER) spectra display a "fast" feature, centred at TKER approximately 9200 cm(-1). The analysis of the structure evident in the fast feature reveals the selective population of specific in-plane stretching vibrational levels of the imidazolyl cofragment; these fragments are deduced to carry only modest amounts of rotational excitation. Comparison with calculated normal mode vibrational frequencies allows the assignment of the populated levels and a precise determination of the N-H bond strength in imidazole: D(0)=33,240+/-40 cm(-1). The observed energy disposal can be rationalized using Franck-Condon arguments, assuming that the potential energy surface (PES) for the 1 (1)A(")((1)pisigma(*)) state has a topology similar to that of the corresponding (1)pisigma(*) state of pyrrole. As in pyrrole, photoexcitation populates skeletal motions in the S(1) state (in-plane motions in the present case) that are only weakly coupled to the N-H dissociation coordinate and thus map through into the corresponding product vibrations. A second, "slow" feature is increasingly evident in TKER spectra recorded at shorter lambda(phot). This component, which exhibits no recoil anisotropy, is attributed to H atoms formed by the "statistical" decay of highly vibrationally excited ground state molecules. The form of the TKER spectra observed at short lambda(phot) is rationalized by assuming two possible decay routes for imidazole molecules excited to the 2 (1)A(')((1)pipi(*)) state. One involves fast 2 (1)A(')((1)pipi(*)) right arrow-wavy 1 (1)A(")((1)pisigma(*)) radiationless transfer and subsequent fragmentation on the 1 (1)A(')((1)pisigma(*)) PES, yielding fast H atoms (and imidazolyl cofragments)-reminiscent of behavior seen at longer excitation wavelengths where the 1 (1)A(")((1)pisigma(*)) PES is accessed directly. The second is assumed to involve radiationless transfer to the ground state, most probably by successive 2 (1)A(') right arrow-wavy 1 (1)A(") right arrow-wavy X (1)A(') couplings, mediated by conical intersections between the relevant PESs and the subsequent unimolecular decay of the resulting highly vibrationally excited ground state molecules yielding slow H atoms.     

High resolution photofragment translational spectroscopy studies of the near ultraviolet photolysis of 2,5-dimethylpyrrole

The photodissociation dynamics of 2,5-dimethylpyrrole (2,5-DMP) has been investigated following excitation at 193.3 nm and at many near ultraviolet (UV) wavelengths in the range 244 < lambda(phot) < 282 nm using H Rydberg atom photofragment translational spectroscopy (PTS). Complementary UV absorption and, at the longest excitation wavelengths, one photon resonant multiphoton ionisation spectra of 2,5-DMP are reported also; analysis of the latter highlights the role of methyl torsional motions in promoting the parent absorption. The deduced fragmentation dynamics show parallels with that reported recently (B. Cronin, M. G. D. Nix, R. H. Qadiri and M. N. R. Ashfold, Phys. Chem. Chem. Phys., 2004, 6, 5031) for the bare pyrrole molecule. Excitation at the longer wavelengths leads to (vibronically induced) population of the 1(1)A(2)(pisigma*) excited state of 2,5-DMP, but once lambda(phot) decreases to approximately 250 nm stronger, dipole allowed transitions start to become apparent in the parent absorption. All total kinetic energy release (TKER) spectra of the H + 2,5-dimethylpyrrolyl (2,5-DMPyl) fragments measured at lambda(phot)> or=244 nm show a structured fast component, many of which are dominated by a peak with TKER approximately 5100 cm(-1); analysis of this structure reveals lambda(phot) dependent population of selected vibrational levels of 2,5-DMPyl, and enables determination of the N-H bond strength in 2,5-DMP: D(0) = 30 530 +/- 100 cm(-1). Two classes of behaviour are proposed to account for details of the observed energy partitioning. Both assume that N-H bond fission involves passage over (or tunnelling through) a small exit channel barrier on the 1(1)A(2) potential energy surface, but differ according to the vibrational energy content of the photo-prepared molecules. Specific parent out-of-plane skeletal modes that promote the 1(1)A(2)-X(1)A(1) absorption appear to evolve adiabatically into the corresponding vibrations of the 2,5-DMPyl products. Methyl torsions can also promote the 1(1)A(2)<-- X(1)A(1) absorption in 2,5-DMP, and provide a means of populating a much higher density of excited vibrational levels than in pyrrole. Such excited levels are deduced to dissociate by redistributing the minimum amount of internal energy necessary to overcome the exit channel barrier in the N-H dissociation coordinate. Coupling with the ground state surface via a conical intersection at extended N-H bond lengths is proposed as a further mechanism for modest translational --> vibrational energy transfer within the separating products. The parent absorption cross-section increases considerably at wavelengths approximately 250 nm, and PTS spectra recorded at lambda(phot)< or = 254 nm display a second, unstructured, peak at lower TKER. As in pyrrole, this slower component is attributed to H atoms from the unimolecular decay of highly vibrationally excited ground state molecules formed via radiationless decay from photo-excited states lying above the 1(1)A(2) state.     

High resolution photofragment spectroscopy of the gas phase methyl iodide ion

Photodissociation spectra of the molecular ion CH₃I⁺ were obtained with a three stage quadrupole mass spectrometer. Starting from the \(\tilde X^2 E_{3/2} \) ground state, theà ² E _1/2 state was excited with a stilbene 3 cw dye laser. This state predissociates to CH ₊ ³ +I. Measuring the intensity of the CH ₊ ³ fragment ions as a function of the wavelength of the exciting laser, a spectrum showing vibrational and rotational structure was obtained. The vibrational structure was assigned to three progressions ofv ₃ and new vibrational frequencies were determined. From a computer simulation of the (0, 1, 10) band rotational constants were derived. In particular, their dependence on the vibrationv ₃ was studied.     

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.     

H atom elimination from the pi sigma* state in the photodissociation of phenol

Photodissociation of phenol at 248 nm was studied using multimass ion imaging techniques. Photofragment translational energy distribution of H atom elimination was measured. The results demonstrate that H atom elimination occurs on the pi sigma(*) excited state which has repulsive potential-energy functions with respect to the stretching of OH bond. It supports the recent ab initio calculation.     

Time-dependent quantum wave-packet description of the 1pi sigma* photochemistry of phenol

The photoinduced hydrogen elimination reaction in phenol via the conical intersections of the dissociative 1pi sigma* state with the 1pi pi* state and the electronic ground state has been investigated by time-dependent quantum wave-packet calculations. A model including three intersecting electronic potential-energy surfaces (S0, 1pi sigma*, and 1pi pi*) and two nuclear degrees of freedom (OH stretching and OH torsion) has been constructed on the basis of accurate ab initio multireference electronic-structure data. The electronic population transfer processes at the conical intersections, the branching ratio between the two dissociation channels, and their dependence on the initial vibrational levels have been investigated by photoexciting phenol from different vibrational levels of its ground electronic state. The nonadiabatic transitions between the excited states and the ground state occur on a time scale of a few tens of femtoseconds if the 1pi pi*-1pi sigma* conical intersection is directly accessible, which requires the excitation of at least one quantum of the OH stretching mode in the 1pi pi* state. It is shown that the node structure, which is imposed on the nuclear wave packet by the initial preparation as well as by the transition through the first conical intersection (1pi pi*-1pi sigma*), has a profound effect on the nonadiabatic dynamics at the second conical intersection (1pi sigma*-S0). These findings suggest that laser control of the photodissociation of phenol via IR mode-specific excitation of vibrational levels in the electronic ground state should be possible.     

High resolution photoabsorption and photofragment fluorescence spectroscopy of water between 10.9 and 12 eV

This work presents absorption and photofragment fluorescence spectra of water (H2O and D2O) simultaneously recorded at rotational resolution and at room temperature, by means of a synchrotron radiation source in the range 10.9-12 eV, covering the nd intense series from n=3 to 8. The Rydberg states observed are assigned in the light of the most advanced theoretical work available [M. S. Child, Philos. Trans. R. Soc. London, Ser. A 355, 1623 (1997)], and by reference to the stretching and bending mode progressions. Comparison between absorption and fluorescence spectra is shown to reveal a fast predissociation mechanism involving the linear 3pb2 1B2 state, and permits the identification of its (0,14,0) vibrational level observed in the absorption spectra.     

Ultrafast photodissociation dynamics of acetone at 195 nm: II. Unraveling complex three-body dissociation dynamics by femtosecond time-resolved photofragment translational spectroscopy

As a continuation of the preceding paper in this issue (J. Phys. Chem. A 2005, 109, 6805), we studied photodissociation dynamics of the acetone S2 (n, 3s) Rydberg state excited at 195 nm using femtosecond time-resolved photofragment translational spectroscopy. The technique, which is implemented by the combination of fs pump-probe ionization spectroscopy and kinetic energy resolved time-of-flight mass spectrometry (KETOF), measured temporal evolutions of the product kinetic energy distributions (KEDs) with a time resolution limited only by the laser pulse widths. Two methyl product KED components were resolved and assigned to the primary and secondary methyl products on the basis of their temporal behaviors. The results support the mechanism in which the primary dissociation occurs on the acetone S1 surface and provide complementary dynamical information to that discussed in the preceding paper.     

High-resolution photofragment translational spectro- scopy for the UV photodissociation of C_2H_5I

The photodissociation of ethyl iodide at 279.71, 281.73, 304.02 and 304.67 nm has been studied on our new mini-photofragment translational spectrometer with a total flight path of only 5 cm. Some vibra-tional peaks are firstly resolved in the TOF spectra of I*(2P1/2) and I(2P3/2) channels. These vibrational peaks are assigned to the excitation states (ν2 = 0, 1, 2,…) of the umbrella mode (ν2, 540 cm-1) of the photofragment C2H5, and the distribution of the vibrational states is obtained. The dissociation energy has been determined to be D0(C-I)=2.314 ± 0.03 eV. The energy partitioning of the available energy (Eavl=ET Eint=ET EV,R) calculated from our experimental data E int /E avl= 22.1% at 281.73 nm, 22.4% at 304.02 nm for the I* channel, and E int /E avl= 25.2% at 279.71 nm, 25.9% at 304.67 nm for the I channel, seem to be more reliable.     

Ultrafast decay of electronically excited singlet cytosine via a pi,pi* to n(O),pi* state switch

Singlet fluorescence lifetimes of adenosine, cytidine, guanosine, and thymidine, determined by femtosecond pump-probe spectroscopy (Pecourt, J.-M. L.; Peon, J.; Kohler, B. J. Am. Chem. Soc. 2000, 122, 9348. Pecourt, J.-M. L.; Peon, J.; Kohler, B. J. Am. Chem. Soc. 2001, 123, 10370), show that the excited states produced by 263 nm light in these nucleosides decay in the subpicosecond range (290-720 fs). Ultrafast radiationless decay to the ground state greatly reduces the probability of photochemical damage. In this work we present a theoretical study of isolated cytosine, the chromophore of cytidine. The experimental lifetime of 720 fs indicates that there must be an ultrafast decay channel for this species. We have documented the possible decay channels and approximate energetics, using a valence-bond derived analysis to rationalize the structural details of the paths. The mechanism favored by our calculations and the experimental data involves (1) a two-mode decay coordinate composed of initial bond length inversion followed by internal vibrational energy redistribution (IVR) to populate a carbon pyramidalization mode, (2) a state switch between the pi,pi* and nO,pi* (excitation from oxygen lone pair) excited states, and (3) decay to the ground state through a conical intersention. A second decay path through the nN,pi* state (excitation from the nitrogen lone pair), with a higher barrier, involves out-of-plane bending of the amino substituent.     

Energy levels of iodine-129 near the B state dissociation limit from two-photon spectroscopy

Two-photon sequential absorption spectroscopy has been used to selectively excite rotational structure in the B---X absorption transition of the ¹²⁹I₂ molecule up to within 2 cm⁻¹ of the dissociation limit. Vibrational and rotational constants have been obtained for the B state levels with υ = 71–79. Le Roy's equations for long-range behaviour are expressed in mass-reduced form, and it is shown that these equations can be successfully used to combine the spectroscopic constants for ¹²⁷I₂ and ¹²⁹I₂ to give improved values of the long range constants for the B state. The improved values are only slightly different from those previously obtained by the same technique for ¹²⁷I₂ alone.     

The role of pi sigma* state in intramolecular electron-transfer dynamics of 4-dimethylaminobenzonitrile and related molecules

Evidence is presented which indicates that the photoinduced intramolecular charge transfer (ICT) in 4-dimethylaminobenzonitrile proceeds by a new mechanism in which pi sigma(C triple bond N) (*) state is the intermediate of a consecutive process that takes the initially excited pi pi(*) state to the fully charge-separated ICT state. The absence of the ICT-state formation in 4-aminobenzonitrile is attributed to the smaller electron-donor strength of the amino group relative to the dimethylamino group, which hinders the pi sigma(*)-->ICT charge-shift reaction.     

The different photoisomerization efficiency of azobenzene in the lowest n pi* and pi pi* singlets: the role of a phantom state

Azobenzene E<==>Z photoisomerization, following excitation to the bright S(pi pi*) state, is investigated by means of ab initio CASSCF optimizations and perturbative CASPT2 corrections. Specifically, by elucidating the S(pi pi*) deactivation paths, we explain the mechanism responsible for azobenzene photoisomerization, the lower isomerization quantum yields observed for the S(pi pi*) excitation than for the S1(n pi*) excitation in the isolated molecule, and the recovery of the Kasha rule observed in sterically hindered azobenzenes. We find that a doubly excited state is a photoreaction intermediate that plays a very important role in the decay of the bright S(pi pi*). We show that this doubly excited state, which is immediately populated by molecules excited to S(pi pi*), drives the photoisomerization along the torsion path and also induces a fast internal conversion to the S1(n pi*) at a variety of geometries, thus shaping (all the most important features of) the S(pi pi*) decay pathway and photoreactivity. We reach this conclusion by determining the critical structures, the minimum energy paths originating on the bright S(pi pi*) state and on other relevant excited states including S1(n pi*), and by characterizing the conical intersection seams that are important in deciding the photochemical outcome. The model is consistent with the most recent time-resolved spectroscopic and photochemical data.     

Photophysics of aromatic molecules with low-lying pi sigma* states: fluorinated benzenes

Unlike fluorinated benzenes with four or less fluorine atoms, pentafluorobenzene (PFB) and hexafluorobenzene (HFB) exhibit very small fluorescence yields and short fluorescence lifetimes. These emission anomalies suggest that the nature of the first excited singlet (S(1)) state may be different for the two classes of fluorobenzenes. Consistent with this conjecture, the time-dependent density-functional theory calculations yield S(1) state of pi pi(*) character for fluorinated benzenes with four or less F atoms, and S(1) state of pi sigma(*) character for PFB and HFB. The pi sigma(*) character of the S(1) state of PFB and HFB has been confirmed by laser-induced fluorescence, which reveal the presence of a new electronic transition to the red of the (1)pi pi(*) (L(b))<--S(0) transition, which can be identified with the predicted low-energy (1)pi sigma(*)<--S(0) absorption. The low fluorescence yields and the short fluorescence lifetimes of PFB and HFB are consistent with the small radiative decay rate of the (1)pi sigma(*) state and efficient S(1) (pi sigma(*))-->S(0) internal conversion between two electronic states of very different geometries.     

High resolution spectroscopy and potential determination of the (3)1Pi state of NaCs

The (3)(1)Pi state of the NaCs molecule was studied by high resolution Fourier-transform spectroscopy. The (3)(1)Pi-->X (1)Sigma(+) laser induced fluorescence was excited by an Ar(+) ion laser or by a single-mode frequency-doubled cw Nd:YAG laser. The presence of argon buffer gas yielded rich rotational relaxation spectra allowing to enlarge the data set for the (3)(1)Pi state term values, as well as to observe Lambda splittings in a wide range of vibrational (v(')) and rotational (J(')) quantum numbers. The data field includes about 820 energy levels of (3)(1)Pi NaCs in the range from v(')=0 to 37 and from J(')=3 to 190, which corresponds to ca. 95% of the potential well depth. Direct fit of the potential energy curve to the level energies is realized using the inverted perturbation approach method; a set of Dunham coefficients is also presented.     

Effects of localized triplet exciton on reactivity of photoinduced omega-bond dissociation in naphthyl phenyl ketones having pi,pi* lowest triplet (T1) states studied by laser flash photolysis

Photochemical properties of photoinduced omega-bond dissociation in naphthyl phenyl ketones having a phenylthiyl moiety as a leaving group, p-(alpha-naphthoyl)benzyl phenyl sulfide (NBPS) and 4-benzoyl-1-naphthylmethyl phenyl sulfide (BNMPS), in solution were investigated by laser flash photolysis techniques. Both ketones were shown to undergo photoinduced omega-bond cleavage of the C-S bond to release the phenyl thiyl radical (PTR) at room temperature. Irrespective of excitation wavelengths of NBPS, a quantum yield (Phi(rad)) of the PTR formation was obtained to be 0.1, whereas that for BNMPS was found to depend on the excitation wavelength, i.e., absorption bands from the ground state (S0) to the excited singlet states, S3, S2, and S1 of BNMPS; Phi(rad)(S3) = 0.77 and Phi(rad)(S2) = Phi(rad)(S1) = 1.0. By using triplet sensitization of p-phenylbenzophenone (PBP), efficiencies (alpha(rad)) of the radical formation in the lowest triplet state (T1(pi,pi*)) of NBPS and BNMPS were determined to be 0 and 1.0, respectively. The agreement between Phi(rad)(S1) and alpha(rad) values for BNMPS indicates that the C-S bond dissociation occurs in the T1 state via the S1 state via a fast intersystem crossing from the S1 to the T1 state. The wavelength dependence of the radical yields upon direct excitation of BNMPS was interpreted in terms of the C-S bond cleavage in the S3 state competing with internal conversion from the S3 to the S2 state. The smaller value of Phi(rad)(S3) than those of Phi(rad)(S1) and Phi(rad)(S2) was proposed to originate from the geminate recombination of singlet radical pairs produced by the bond dissociation via the S3 state. Photoinduced omega-cleavage of NBPS was concluded to take place only in the S1(n,pi*) state. Difference in reactivity of omega-cleavage between the triplet states of NBPS and BNMPS was interpreted in terms of localized triplet exciton in the naphthoyl moieties.     

Spectroscopy and photophysics of 1,4-bis(phenylethynyl)benzene: effects of ring torsion and dark pi sigma* state

A combination of supersonic-jet laser spectroscopy and quantum chemistry calculation was applied to 1,4-bis(phenylethynyl)benzene, BPEB, to study the role of the dark pisigma* state on electronic relaxation and the effect of ring torsion on electronic spectra. The result provides evidence for fluorescence break-off in supersonic jet at high S1(pi pi*) <-- S0 excitation energies, which can be attributed to the pi pi*-pi sigma* intersection. The threshold energy for the fluorescence break-off is much larger in BPEB (approximately 4000 cm(-1)) than in diphenylacetylene (approximately 500 cm(-1)). The high-energy barrier in BPEB accounts for the very large fluorescence quantum yield of the compound (in solution) relative to diphenylacetylene. The comparison between the experimentally derived torsional barrier and frequency with those from the computation shows overall good agreement and demonstrates that the low-energy torsional motion involves the twisting of the end ring in BPEB. The torsional barrier is almost an order of magnitude greater in the pi pi* excited state than in the ground state. The finding that the twisting of the end ring in BPEB is relatively free in the ground state, but strongly hindered in the excited state, provides rationale for the well-known temperature dependence of the spectral shape of absorption and the lack of mirror symmetry relationship between the absorption and fluorescence at elevated temperatures.     

Photochemistry of pyrrole: time-dependent quantum wave-packet description of the dynamics at the 1pi sigma*-S0 conical intersections

The photoinduced hydrogen-elimination reaction in pyrrole via the conical intersections of the two (1)pi sigma(*) excited states with the electronic ground states [(1)B(1)(pi sigma(*))-S(0) and (1)A(2)(pi sigma(*))-S(0)] have been investigated by time-dependent quantum wave-packet calculations. Model potential-energy surfaces of reduced dimensionality have been constructed on the basis of accurate multireference ab initio electronic-structure calculations. For the (1)B(1)-S(0) conical intersection, the model includes the NH stretching coordinate as the tuning mode and the hydrogen out-of-plane bending coordinate as the coupling mode. For the (1)A(2)-S(0) conical intersection, the NH stretching coordinate and the screwing coordinate of the ring hydrogens are taken into account. The latter is the dominant coupling mode of this conical intersection. The electronic population-transfer processes at the conical intersections, the branching ratio between the dissociation channels, and their dependence on the initial preparation of the system have been investigated for pyrrole and deuterated pyrrole. It is shown that the excitation of the NH stretching mode strongly enhances the reaction rate, while the excitation of the coupling mode influences the branching ratio of different dissociation channels. The results suggest that laser control of the photodissociation of pyrrole via mode-specific vibrational excitation should be possible. The calculations provide insight into the microscopic details of ultrafast internal-conversion processes in pyrrole via hydrogen-detachment processes, which are aborted at the (1)pi sigma(*)-S(0) conical intersections. These mechanisms are of relevance for the photostability of the building blocks of life (e.g., the DNA bases).