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We report a picosecond time-resolved photoionization and photoelectron spectroscopic study of acenaphthylene, C 12 H 8 in a free jet. The origin for the A 1 B 2 (S 1 ) ← X 1 A 1 (S 0 ) transition is found at 21,757 ± 15 cm −1 , while the one for the B 2 A 1 ← X 1 A 1 transition is observed at 30,076 ± 27 cm −1 . For both transitions vibrational bands are resolved and assigned. For the origin of the S 1 state a lifetime of 380 ps is measured, which decreases with excess energy. Internal conversion to the electronic ground state seems to be the dominant deactivation pathway. For the S 2 state an additional component with a very short lifetime of 1 ps or less is observed. This indicates a two-step deactivation via S 1 to the electronic ground state. A small and unstructured signal in the photoelectron spectra at long delay times suggests the presence of an additional deactivation channel, most likely intersystem crossing.

Indigo is a blue dye molecule that has been used since antiquity, although it is better known today for its use in blue jeans. Indigo has previously been shown to exhibit remarkable photostability due to fast excited state dynamics mediated by an excited state intramolecular proton transfer. Study of this process is complicated by the fact that the photophysics of indigo is very sensitive to the environment. In order to disentangle the intrinsic photodynamics of indigo from the effects contributed by the environment, we studied indigo in a molecular beam using resonance enhanced multiphoton ionization. We obtained excited state lifetimes of individual vibronic bands with pump-probe spectroscopy ranging from 60 ps to 24 ns. We have mapped a barrier to relaxation at about 700 cm−1, beyond which fast excited state dynamics dominate. Below this barrier two decay processes compete and mode-specific relaxation occurs with certain vibronic bands near the origin relaxing faster than others, or exhibiting different partitioning between the two relaxation channels. Computational studies at the ADC(2)/MP2/cc-pVDZ level indicate that two low-barrier reaction paths exist in the S1 state of indigo, one corresponding to proton transfer, the other to hydrogen transfer. In both cases the charge distribution changes drastically upon de-excitation. These data provide a sensitive probe of the potential energy landscape, responsible for the response to absorption of light. The results may help in understanding the photostability that preserves the blue color of indigo dyes.

Photoionization of the hydrogen atom by intense and short coherent laser pulses is investigated from first principles by a numerical solution of the time-dependent Schrodinger equation in the dipole-velocity gauge. The considered photon energies are resonant to the 1 s → 2 p excitation, and the pulse intensities are high enough to induce Rabi floppings. The computed resonance-enhanced two-photon ionization spectra as well as the three-photon above threshold ionization spectra exhibit pronounced multiple-peak patterns due to dynamic interference. Fingerprints of dynamic interference can also be seen directly in the radial density of the photoelectron. The impact of the variation of the pulse intensity and photon energy on the dynamic interference is investigated, and the angular distribution of the emitted electrons is analyzed in some details. The present precise numerical results confirm our previous theoretical predictions on the two-photon ionization spectra (Demekhin and Cederbaum, 2012) made within a minimal few-level model.

The photodissociation dynamics of halogen substituted pyridines, namely, 3-chloropyridine (ClPy) and 3-chloro-2,4,5,6-tetrafluoropyridine (ClFPy), has been studied around 235 nm by detecting chlorine atoms in their spin orbit states Cl( 2 P 3/2 ) and Cl ∗ ( 2 P 1/2 ) using the REMPI-TOF technique. We have determined the translational energy distribution, the recoil anisotropy parameter, β , and the spin–orbit branching ratio, for chlorine atom elimination channels. The TOF profiles for Cl and Cl ∗ are found to be independent of laser polarization suggesting a zero value for β , within the experimental uncertainties. For 3-chloropyridine, the average translational energies for Cl and Cl ∗ elimination channels are determined to be 3.7 ± 1.0 and 7.0 ± 1.5 kcal/mol, respectively. Similarly, for 3-chloro-2,4,5,6-tetrafluoropyridine, the average translational energies for Cl and Cl ∗ elimination channels are determined to be 8.0 ± 1.5 and 9.0 ± 1.5 kcal/mol, respectively. The theoretical calculation suggests that the fluorine substitution increases the possibility of cross over to the π – σ ∗ state from the initially prepared π – π ∗ state.

Conformer selected electronic and vibrational spectra of homophenylalanine, phenylalanine analogue molecule, were measured by UV–UV hole burning and IR dip spectroscopy combined with laser desorption technique. 10 conformers were found by UV–UV hole burning spectroscopy and their structures were assigned by IR dip and UV absorption spectra with aid of quantum chemical calculations in both S 0 and S 1 . This study shows that the combination of simulated IR and UV spectra is powerful to assign flexible molecules.

In one-color REMPI-TOF experiment, the photodissociation dynamics of tetrachloroethylene has been studied by probing the chlorine atom photofragments, namely, Cl (2P3/2) and Cl∗ (2P1/2), using 2 + 1 REMPI scheme in the 234–236 nm region. We have determined the centre-of-mass photofragment speed distribution, recoil anisotropy parameter (β), and the spin–orbit branching ratio for chlorine atom elimination channels. The β is well characterized by a value of ∼0.0, within the experimental uncertainties. Two components, namely, the fast and the slow, are observed in the translational energy distributions of Cl and Cl∗. The average translational energies for the Cl and Cl∗ channels for the fast components are 17.6 ± 1.9 and 14.0 ± 1.7 kcal/mol, while, that for the slow components are 2.2 ± 1.0 and 3.2 ± 1.0 kcal/mol, respectively. To understand the nature of the dissociative potential energy surface involved in the chlorine atom formation channel, detailed theoretical calculations are performed using Time-dependent Density Functional Theory (TD-DFT) method.

In one-color REMPI-TOF experiment, the photodissociation dynamics of POCl 3 has been studied by photolyzing POCl 3 and probing the chlorine atom photofragments, namely, Cl( 2 P 3/2 ) and Cl ∗ ( 2 P 1/2 ) using 2 + 1 REMPI scheme, in the 234–236 nm region. We have determined the centre-of-mass photofragment speed distribution, recoil anisotropy parameter, and the spin–orbit branching ratio for chlorine atom elimination channels. The anisotropy parameters for Cl and Cl ∗ are the same, and characterized by a value of 0.0 ± 0.05. Two components, namely, the fast and the slow, are observed in the translational energy distributions of Cl and Cl ∗ . The average translational energies for the Cl and Cl ∗ channels for the fast components are 12.5 ± 1.5 and 16.8 ± 1.5 kcal/mol, while, for the slow components, the average translational energies are 1.5 ± 1.0 and 2.5 ± 1.0 kcal/mol, respectively. Apart from the chlorine atom elimination channel, Cl 2 elimination is also observed in the photodissociation of POCl 3 .

Mass-resolved, resonance-enhanced multiphoton ionization (REMPI) spectra of jet-cooled 2-methylfuran have been measured in various spectroscopic regions (visible and UV ns dye laser). The results have been interpreted as two-photon resonances and as three-photon resonances to the 3d Rydberg manifold chiefly. The 3d 1 A 1 ← ( X ∼ ) A 1 transition, located at 56,360 cm−1, is the strongest one in the (2 + 1) REMPI spectrum and has a discrete vibrational structure. The vibrational analysis has been performed. The three-photon resonance spectrum tracks the 3d VUV absorption spectrum. The strongest transition in this case is the 3d 1 B 2 ← ( X ∼ ) A 1 transition located at 56,013 cm−1. The transition to the 3s A2 Rydberg state has been detected at 44,018 cm−1 (3 + 2 REMPI spectrum).

The photodissociation of CH2Br2, 1,2-C2H4Br2 and 1,4-C4H8Br2 near 234 and 267 nm has been studied using resonance-enhanced multiphoton ionization with time-of-flight mass spectrometry. The wavelength dependence of the relative quantum yields Phi(Br), and Phi(Br*) for both Br(P-2(3/2)0) and Br*(P-2(1/2)0) channels was measured. The results indicated that the yields Phi(Br) of C2H4Br2 is larger than that of CH2Br2. And the quantum yields wavelength dependent of C4H8Br2 shows a different tendency compared to CH2Br2 and C2H4Br2. A likely explanation for the different quantum yields relies on the non-adiabatic transitions and avoided curve crossing mechanism. Additionally, ab initio calculations were performed to get potential energy surface curves of CH2Br2 to elucidate the photodissociation channels in UV region. (c) 2005 Elsevier B.V. All rights reserved.

Slice imaging is used to measure the velocity distribution of the H-atom photofragments from the photodissociation of HBr at 243 nm. The achievable velocity resolution of our measurements is improved by a factor of 1.5 when (3 + 1) resonance enhanced multiphoton ionization (REMPI) via the 2p-excited state is used rather than the popular (2 + 1) REMPI via the 2s-excited state. This observation is attributed to the large difference in the recoil momentum imparted to the H + ion by the sibling photoelectron during the ionization process. Although the efficiency of the (3 + 1) REMPI is substantially less than the (2 + 1) REMPI, the line-width of the former transition is sufficiently broad to measure the complete angular distribution of the H-atom products, without scanning the probe laser wavelength. From the H-atom images, the Br( 2 P 1/2 )/Br( 2 P 3/2 ) branching ratio is found to be 0.11.

We study the detection of collision induced alignment in ensembles of NO molecules scattered from He through (2 + 1) resonance enhanced multiphoton ionization (REMPI) of NO via the Rydberg states: H 2 Σ , H ′ 2 Π . The two-photon spectroscopy of the H–X transition is complicated by the state mixing due to L -uncoupling and the simultaneous presence of zeroth and second rank tensor components for this strong transition. The creation of an aligned ensemble of NO molecules in a well-defined collision process enables us to study in detail the polarization effect for different branches of the H–X two-photon spectrum. The polarization effect results from the interference of the second rank tensor components with the zeroth rank tensor of the two-photon absorption tensor. Variations in the polarization effect for perturbed levels of the excited state reflect directly the changes in the expansion coefficients for the involved electronic wavefunctions. The experimentally observed polarization effects are consistent with the non-vanishing components ( Z (0) 0 : Z (2) 0 : Z (2) 2 )=(1.0:−0.2:0.6) of the two-photon absorption tensors.

The detection of collision induced alignment in ensembles of NO molecules scattered from He or Ne is investigated. The alignment is probed through (2 + 1) resonance enhanced multiphoton ionization of NO via different Rydberg states: E 2 Σ , F 2 Δ , and H 2 Σ , H ′ 2 Π . Transitions belonging to the first two electronic systems yield identical polarization effects consistent with the fact that the two-photon transition is characterized by a single second rank tensor component Z (2) ±1 . The electronic system involving the H-state is considerably more complicated because of the state mixing due to L-uncoupling and the simultaneous presence of zeroth and second rank tensor components for the two-photon transition. Alignment moments extracted from TOF spectra measured through different electronic systems are identical within the experimental uncertainty. The alignment is consistent with predictions of either the kinematic apse or the geometric apse models. These models indicate further that higher moments have a negligible influence on the polarization effect if the alignment is derived from spectra measured with laser light polarized parallel and perpendicular to the molecular beam axis.

(1+1′) resonance enhanced multiphoton ionization (REMPI) and dispersive photoelectron (PE) spectrometry have been used to identify the ionic cores associated with ungerade Xe2 Rydberg states dissociating to Xe ( 1 S 0 )+( Xe * 6 p [1/2] 0 , Xe * 5 d [5/2] 3 0 , and Xe*[3/2]10). Several features in the PE spectra were also found to originate from the ionization of excited Xe atom terms produced by molecular predissociation. These results support a previous REMPI / time-of-flight mass spectrometry study on the excited states considered here, and confirm predictions made from angular momentum arguments.

The (2+1) resonance enhanced multiphoton ionization spectra of HBr were obtained through the E 1 Σ + state for HBr seeded in Ar free jets. Analysis of the two-photon absorption line strength factors allowed determination of quantitative populations for the HBr rotational manifold. A study of the terminal HBr rotational state distributions was made for a wide variety of p 0 D values of the jets. These nonequilibrium population distributions were then modeled by numerical solutions of the set of coupled kinetic equations describing the rotationally inelastic collisions with the buffer. Using a power-law relaxation model, the j – j ′ rotationally state specific relaxation cross-sections and rate coefficients were determined as a function of temperature over the range present in the post-continuum zone of the free jet flows (180 to 15 K).

Ar–NO in the C and D states, which are correlated to two components of 3p Rydberg complex of NO, C 2 Π and D 2 Σ + , were studied by using a mass-resolved (2+1) or (1+1 ′ ) resonance enhanced multiphoton ionization (REMPI) technique. The REMPI excitation spectra were measured by monitoring parent Ar–NO + or fragment NO + . The binding energies and the vibrational parameters are determined for the Ar–NO C and D states. The D state was found to have surprisingly large D 0 values of 1044 cm −1 for v NO ′ =0 and 1004 cm −1 for v NO ′ =1, which are larger than 940 cm −1 for Ar–NO + . From the analysis of these spectra, it was found that Ar–NO in the 3p Rydberg states predissociates into NO and Ar at different rates strongly depending on the Rydberg orbital character, the vibrational quantum number ( v NO ′ ) of NO moiety, and the intermolecular stretching quantum number ( v str ′ ) of Ar–NO. Especially, the interaction of the 3p Rydberg states with the nearest-neighboring B state plays an important role in these v NO ′ - and v str ′ -dependent predissociation of Ar–NO C( v NO ′ =0, 1, 2) and D( v NO ′ =0, 1).

The observation of rare gas cations (Rg + ) in the multiphoton ionization of Rg · NO is studied. Kr + and Xe + are produced when exciting via the A state of Rg · NO, addition to Rg · NO + . In contrast, only Xe + is seen when exciting via the C state of Xe · NO, and only Kr · NO + is seen when exciting via the C state of Kr · NO. Ar + is not seen when exciting via either the A or C states of Ar · NO. The production of the different ions is studied as far of laser power by altering the laser intensity and focusing conditions. For excitation via the A state of Kr · NO, it is found that Kr + production is favoured by high laser intensities, with Kr · NO + being favoured by low intensities. Under tight focusing conditions for the one-colour experiments on Kr · NO, Kr + ions are also produced on NO A ← X resonances. Two-colour experimenrs are also performed on Kr · NO. The observation of Kr + on Kr · NO resonances is explained by intracomplex photoinduced charge transfer within the core of a Rydberg state, with collisional charge transfer between NO + and Kr · NO also playing a role. The production of Kr + on NO resonances is explained by photoinduced collisional charge transfer.

The UV photodesorption (6.4 eV) of ammonia from Cu(111) has been investigated using resonance enhanced multiphoton ionization through the B«rrX transition for state specific detection. We have measured the relative populations of both inversion doublet states of the first three vibrational levels in the v 2 mode and the rotational excitation of photodesorbed NH 3 . The average energy content in the v 2 mode corresponds to a temperature of T v 2 = 1000 K which is significantly larger than the translational energy released normal to the surface Ě E trans > 2k = 600 K . A marked underpopulation of the antisymmetric v 2 levels has been observed. The rotations can be described by a Boltzmann distribution with a temperature close to the surface temperature indicating that the molecules do not experience significant torque during the desorption process. No pronounced correlation effects in the population of different degrees of freedom are found. These experimental results are consistent with the assumption that the energy required for desorption is transferred from the initial electronic excitation to the molecule-surface bond via the intramolecular coordinate v 2 .

Two striking examples of Rydberg/ion-pair state coupling have been observed in the spectrum of I 2 in the regions 62500–64000 and 56500–59500 cm −1 . The coupling is of intermediate strength and the (2+1) REMPI spectra of the resonant Ry ← X transitions have ion-pair satellites around each Rydberg vibrational level from which they have borrowed intensity. The Rydberg states are identified as the [ 2 Π 3 2 ] c 7 s ;1 g and [ 2 Π 3 2 ] c 5 d ; 2 g and the ion-pair states as the β1g and D′2g. Curve crossings occur close to the ν ′ = 532 level of the β state and ν ′ = 311 of the D′ state respectively. The spectra are modelled by solving the two-state coupling problem in a diabatic basis and in order to deduce approximate values of the coupling matrix elements, extrapolations of the known portions of the potential energy curves of the β1g and D′2g ion-pair states are necessary. The diagnostic value of the magnitude of the electronic part of Rydberg/ion-pair coupling matrix elements in probing the electronic configuration of ion-pair states at short range is discussed.

In this work the S 2 molecule has been studied using resonance enhanced multiphoton ionization (MPI) spectroscopy in the single-photon wavelength region 285–320 nm. This has resulted in the first spectroscopic observation of a gerade Rydberg state of S 2 . The observed bands, which show spin—orbit structure, have been assigned to a two-photon resonant process, 3 Δ g (3dπ←X 3 Σ − g . Transitions to three vibrational levels (ν′ = 0, 1, 2) of the excited state have been recorded. This has enabled determination of vibrational constants ( ω e and ω e χ e ) and the spin—orbit coupling constant of the 3 Δ g state, whilst rotational constants ( B ν′ ) were derived from band head separations. A band in the MPI spectrum of S 2 corresponding to the one-photon resonance B 3 Σ − u , ν′=8←X 3 Σ − g , ν″=0 has also been observed. This has been detected in both the S + 2 and S + mass channels. Possible mechanisms for production of these ions via the B 3 Σ − u state are discussed.

The S1 ← S0 origin band of ClC6D5 and the 6a10 bands of 1,4-35Cl2 12C6H4, 1,4-35Cl37Cl12C6H4 and 1,4-37Cl212C6H4 were recorded by the technique of resonance enhanced multiphoton ionization combined with time-of-flight mass spectrometry. The corresponding rotational band contours were fitted to computer calculated synthetic contours leading in each case to the determination of the excited state rotational constants and the band vibronic origins. In the case of the p-dichlorobenzenes, the latter values combined with the 000 band origin obtained by Cvitas and Hollas, allow the determination of ν′6a as 728.0, 726.6 and 726.7 ± 0.4 cm−1 respectively for 35Cl2, 35Cl37Cl and 37Cl2C6H4.

The technique of resonance-enhanced multiphoton ionization has been used to prepare the chlorobenzene cation in a series of different non-equilibrium rotational population distributions in its ground state. The ion is probed by a second laser leading to dissociation into fragments detected mass spectrometrically. The varied rotational contours resulting from this experiment provide valuable extra information for characterization of the ion transition.

Two-photon transitions A 2 Σ + (ν′ = 0)-X 2 Π(ν″ = 0) of nitric oxide are investigated by high-resolution resonance-enhanced multiphoton ionization spectroscopy. The underlying causes behind observed anomalous intensity features of these two-photon transitions are discussed. Several new lines produced by the three-photon transitions “L 2 Π(ν = 4)-C 2 Π(ν = 6)” -X 2 Π(ν″ = 0) and K 2 Π(ν = 1) -X 2 Π(ν″ = 0) are assigned. The values for T ν and B ν of the K 2 Π(ν = 1) state are determi 66350 cm −1 has been resolved. It is suggested that several lines with anomalous intensity on the O 12 branch of the A 2 Σ + (ν′ = band should be interpreted as the result of an autoionization process rather than as the result of an accidental resonance of a third photon with rotational levels of the “L 2 Π(ν = 4)-C 2 Π(ν = 6) state as previously has been anticipated.

The rotational structure of some vibronic bands of the B←X system of the chlorobenzene cation (φCl + ) has been studied by computation of rotational band contours. For the first time contours for ions prepared by resonance enhanced multiphoton ionization (REMPI), with a restricted (or non-equilibrium) rotational excitation have been analysed. Using the mass selectivity of the REMPI method, the relative contributions from the two principal isotopes 35 Clφ and 37 Clφ have been taken into account. For this purpose the excited state rotational constants of 37 Clφ have been determined. The ionic bands are shown to be A type which agrees with our vibrational assignments.

A pulsed tunable dye laser is used for (2 + 1) REMPI in NH 3 via its C′ state with ν 2 = 2. The spectra, in the 3040-3062 A range, originate in 300 K NH 3 . Both circular and linear laser polarization is used. The results are interpreted via the formalism of McClain and Harris. Because (a) we prepare pure light polarization, (b) the transition symmetry is known, and (c) individual rotational lines are resolved, the interpretation is clear and analysis yields molecular invariants that are otherwise difficult to obtain. Comparison is made with a similar treatment for alkyl iodides in which individual lines were not resolved. Alternatively, the same NH 3 transitions are also observed via the optogalvanic effect in a discharge. Because the REMPI process occurs mainly at the focus of the laser, spatially resolved diagnostics are possible.