Photoionization and photodissociation dynamics of the B 1sigmau + and C 1Piu states of H2 and D2

The photoionization and photodissociation dynamics of H(2) and D(2) in selected rovibrational levels of the B (1)Sigma(u) (+) and C (1)Pi(u) states have been investigated by velocity map ion imaging. The selected rotational levels of the B (1)Sigma(u) (+) and C (1)Pi(u) states are prepared by three-photon excitation from the ground state. The absorption of fourth photon results in photoionization to produce H(2)(+) X (2)Sigma(g)(+) or photodissociation to produce a ground-state H(1s) atom and an excited H atom with n >or= 2. The H(2) (+) ion can be photodissociated by absorption of a fifth photon. The resulting H(+) or D(+) ion images provide information on the vibrational state dependence of the photodissociation angular distribution of the molecular ion. The excited H(n >or= 2) atoms produced by the neutral dissociation process can also be ionized by the absorption of a fifth photon. The resulting ion images provide insight into the excited state branching ratios and angular distributions of the neutral photodissociation process. While the experimental ion images contain information on both the ionic and neutral processes, these can be separated based on constraints imposed on the fragment translational energies. The angular distribution of the rings in the ion images indicates that the neutral dissociation of molecular hydrogen and its isotopes is quite complex, and involves coupling to both doubly excited electronic states and the dissociation continua of singly excited Rydberg states.     

Photodissociation dynamics of N2O at 130 nm: the N2(A 3Sigmau +,B 3Pig)+O(3PJ = 2,1,0) channels

Oxygen Rydberg time-of-flight spectroscopy was used to study the vacuum ultraviolet photodissociation dynamics of N(2)O near 130 nm. The O((3)P(J)) products were tagged by excitation to high-n Rydberg levels and subsequently field ionized at a detector. In agreement with previous work, we find that O((3)P(J)) formation following excitation to the repulsive N(2)O D((1)Sigma(+)) state produces the first two electronically excited states of the N(2) counterfragment, N(2)(A (3)Sigma(u) (+)) and N(2)(B (3)Pi(g)). The O((3)P(J)) translational energy distribution reveals that the overall branching ratio between N(2)(A (3)Sigma(u) (+)) and N(2)(B (3)Pi(g)) formation is approximately 1.0:1.0 for J = 1 and 2, with slightly less N(2)(B (3)Pi(g)) produced in coincidence with O((3)P(0)). The angular distributions were found to be independent of J and highly anisotropic, with beta = 1.5+/-0.2.     

Ion-pair formation dynamics of F(2) at 18.385 eV studied by velocity map imaging method

We studied the ion-pair formation dynamics of F2 at 18.385 eV (67.439 nm) using the velocity map imaging method. It was found that there are two dissociation channels corresponding to production of F(+)((1)D(2)) + F(-)((1)S(0)) and F(+)((3)P(j)) + F(-)((1)S(0)). The measured center-of-mass translational energy distribution shows that about 98% of the dissociation occurs via the F(+)((1)D(2)) channel. The measured angular distributions of the photofragments indicate that dissociation for the F(+)((3)P(j)) channel occurs via predissociation of Rydberg states converging to F(2)(+)(A(2)Pi(u)) and dissociation for the F(+)((1)D(2)) channel involves mainly a direct perpendicular transition into the ion-pair state, or X(1)Sigma(g)(+) --> 2(1)Pi(u), which is also supported by the transition dipole moment calculations .     

Ion-pair dissociation of N2O in the 16.25-16.41 eV: dynamics and electronic structure

The ion-pair dissociation dynamics of N(2)O -->(XUV) N(2)(+)(X (2)Sigma(g)(+), v) + O(-)((2)P(j)) at 16.248, 16.271, 16.389, and 16.411 eV have been studied using the velocity map imaging method and tunable XUV laser. The electronic structures of the ion-pair states have been studied by employing the ab initio quantum chemical calculation. The translational energy distributions and the angular distributions of the photofragments have been measured. The results show that about 40% of available energies are transformed into the translational energies, and the first excited vibrational states are populated most strongly for all four excitation energies. The anisotropy parameters beta are approximately 1. The ab initio calculations at the level of CASSCF6-311++g(3df) show that the equilibrium geometries of the ion-pair states are nonlinear with bond lengths R(N-N) = 1.10 A, R(N-O) = 2.15 A, and bond angle N-N-O = 103 degrees, respectively. The ion-pair states are formed by electron migration from the bonding sigma orbital of N[triple bond]N to the antibonding sigma orbital localized primarily on the O atom. Combining the experimental and theoretical results, it is concluded that the ion-pair dissociation occurs via predissociation of Rydberg states with (1)Sigma(+) symmetry, which converges to the ion-core N(2)O(+)(A (2)Sigma(+)).     

Photoelectron imaging following 2 + 1 multiphoton excitation of HBr

The photodissociation and photoionization dynamics of HBr via low-n Rydberg and ion-pair states was studied by using 2 + 1 REMPI spectroscopy and velocity map imaging of photoelectrons. Two-photon excitation at about 9.4-10 eV was used to prepare rotationally selected excited states. Following absorption of the third photon the unperturbed F (1)Delta(2) and i (3)Delta(2) states ionize directly into the ground vibrational state of the molecular ion according to the Franck-Condon principle and upon preservation of the ion core. In case of the V (1)Sigma(+)(0(+)) ion-pair state and the perturbed E (1)Sigma(+)(0(+)), g (3)Sigma(-)(0(+)), and H (1)Sigma(+)(0(+)) Rydberg states the absorption of the third photon additionally results in a long vibrational progression of HBr(+) in the X (2)Pi state as well as formation of electronically excited atomic photofragments. The vibrational excitation of the molecular ion is explained by autoionization of repulsive superexcited states into the ground state of the molecular ion. In contrast to HCl, the perturbed Rydberg states of HBr show strong participation of the direct ionization process, with ionic core preservation.     

Slow electron velocity-map imaging spectroscopy of the C4H- and C4D- anions

High resolution photodetachment spectra of C4H- and C4D- obtained via slow electron velocity-map imaging (SEVI) are presented. The spectra reveal closely spaced transitions to the neutral 2Sigma+ and 2Pi states which can be distinguished based on the corresponding photoelectron angular distributions. The C4H ground state is confirmed as the X2Sigma+ state, with the excited A2Pi state lying only 213 cm(-1) higher (201 cm(-1) for C4D). The electron affinities (EAs) are slightly revised to EA (C4H)=28,497+/-8 cm(-1) and EA (C4D)=28,478+/-10 cm(-1). Progressions in low frequency bending vibrations are observed in both states, yielding experimental frequencies of nu7=179(169) cm(-1) and nu6=408(392) cm(-1) for the X2Sigma+ state of C4H (C4D), and nu7=220(215)cm(-1) and nu6=446(437) cm(-1) for the A2Pi state.     

Observation of L uncoupling in the 5 1Deltag Rydberg state of Na2

The phenomenon of electronic orbital angular momentum L uncoupled from its internuclear axis has been observed in the sodium dimer using high-resolution cw optical-optical double-resonance spectroscopy. When L uncoupling occurs, the degeneracy of Lambda doubling is removed. In our experiment, the intermediate B (1)Pi(u) state of Na(2) is excited from the thermally populated ground X (1)Sigma(g) (+) state by a single-line Ar(+) laser. Then, a single-mode dye laser is used to probe the Rydberg states from the intermediate state. The signals are detected by monitoring the UV fluorescence from the triplet gerade states back to the a (3)Sigma(u) (+) state via collision energy transfer. Under our experimental resolution, the splitting of Lambda doubling in the 5 (1)Delta(g) state of Na(2) can be measured. A total of 136 rovibronic levels with ef parities have been assigned to the 5 (1)Delta(g) state. The Lambda-splitting constants deduced from these data are q(0)=0.376(90)x10(-4) cm(-1), q(v)=0.114(6)x10(-4) cm(-1), and mu=0.76(33)x10(-8) cm(-1). In general, the Lambda splitting of the Delta states is considerably smaller than that of the Pi states. However, the first-order splitting constants q(0) and q(v) reported here are larger than those in the B (1)Pi(u) state. This is due to the L uncoupling of the Rydberg states.     

Photoinitiated predissociation of the NO dimer in the region of the second and third NO stretch overtones

Photofragment yield spectra and NO(X(2)Pi(1/2,3/2); v = 1, 2, 3) product vibrational, rotational, and spin-orbit state distributions were measured following NO dimer excitation in the 4000-7400 cm(-1) region in a molecular beam. Photofragment yield spectra were obtained by monitoring NO(X(2)Pi; v = 1, 2, 3) dissociation products via resonance-enhanced multiphoton ionization. New bands that include the symmetric nu(1) and asymmetric nu(5) NO stretch modes were observed and assigned as 3nu(5), 2nu(1) + nu(5), nu(1) + 3nu(5), and 3nu(1) + nu(5). Dissociation occurs primarily via Deltav = -1 processes with vibrational energy confined preferentially to one of the two NO fragments. The vibrationally excited fragments are born with less rotational energy than predicted statistically, and fragments formed via Deltav = -2 processes have a higher rotational temperature than those produced via Deltav = -1 processes. The rotational excitation likely derives from the transformation of low-lying bending and torsional vibrational levels in the dimer into product rotational states. The NO spin-orbit state distribution reveals a slight preference for the ground (2)Pi(1/2) state, and in analogy with previous results, it is suggested that the predominant channel is X(2)Pi(1/2) + X(2)Pi(3/2). It is suggested that the long-range potential in the N-N coordinate is the locus of nonadiabatic transitions to electronic states correlating with excited product spin-orbit states. No evidence of direct excitation to electronic states whose vertical energies lie in the investigated energy region is obtained.     

Laser spectroscopic studies of several Rydberg states of MgO

We report extensive spectroscopic measurements of rovibronic transitions from the MgO X 1Sigma+ ground state to the high-energy E 1Sigma+, F 1Pi1, and G 1Pi1 Rydberg states. Perturbations in the E 1Sigma+ and G 1Pi1 states were observed. The Rydberg molecular orbital character of the three states is examined, given ab initio calculations by Thummel et al. [Chem. Phys. 129, 417 (1989)]. It is concluded that the E 1Sigma+ and G 1Pi1 states consist primarily of the MgO+ X 2Pi ionic core, surrounded by 3ppi and 3psigma Rydberg electron clouds, respectively, and that the F 1Pi1 state consists primarily of the MgO+ A 2Sigma+ ionic core surrounded by a 3ppi Rydberg electron cloud. Spectroscopic characterizations of some unassigned vibrational levels of analogous MgO 3Pi2 states in this energy region are also reported.     

Ion imaging study of NO3 radical photodissociation dynamics: characterization of multiple reaction pathways

The photodissociation of NO(3) has been studied using velocity map ion imaging. Measurements of the NO(2) + O channel reveal statistical branching ratios of the O((3)P(J)) fine-structure states, isotropic angular distributions, and low product translational energy consistent with barrierless dissociation along the ground state potential surface. There is clear evidence for two distinct pathways to the formation of NO + O(2) products. The dominant pathway (>70% yield) is characterized by vibrationally excited O(2)((3)Σ(g)(-), v = 5-10) and rotationally cold NO((2)Π), while the second pathway is characterized by O(2)((3)Σ(g)(-), v = 0-4) and rotationally hotter NO((2)Π) fragments. We speculate the first pathway has many similarities to the "roaming" dynamics recently implicated in several systems. The rotational angular momentum of the molecular fragments is positively correlated for this channel, suggesting geometric constraints in the dissociation. The second pathway results in almost exclusive formation of NO((2)Π, v = 0). Although product state correlations support dissociation via an as yet unidentified three-center transition state, theoretical confirmation is needed.     

Intense-field modulation of NO2 multiphoton dissociation dynamics

We report on the dynamics of multiphoton excitation and dissociation of NO(2) at wavelengths between 395 and 420 nm and intensities between 4 and 10 TW cm(-2). The breakup of the molecule is monitored by NO A (2)Sigma(+)n(')=1,0-->X (2)Pi(r)n(")=0 fluorescence as a function of time delay between the driving field and a probe field which depletes the emission. It is found that generation of n(')=0 and 1 NO A (2)Sigma(+) results in different fluorescence modulation patterns due to the intense probe field. The dissociation dynamics are interpreted in terms of nuclear motions over light-induced potentials formed by coupling of NO(2) valence and Rydberg states to the applied field. Based on this model, it is argued that the time and intensity dependences of A (2)Sigma(+)n(')=0-->X (2)Pi(r)n(")=0 fluorescence are consistent with delayed generation of NO A (2)Sigma(+)n(')=0 via a light-induced bond-hardening brought about by the transient coupling of the dressed A (2)B(2) and Rydberg 3ssigma (2)Sigma(g) (+) states of the parent molecule. The increasingly prompt decay of A (2)Sigma(+)n(')=1-->X (2)Pi(r)n(")=0 fluorescence with increasing intensity, on the other hand, is consistent with a direct surface crossing between the X (2)A(1) and 3ssigma (2)Sigma(g) (+) dressed states to generate vibrationally excited products.     

Optical-optical double resonance photoionization spectroscopy of nf Rydberg states of nitric oxide

The spectra of vibrationally excited nf Rydberg states of nitric oxide were recorded by monitoring the photoion current produced using two-photon double resonance excitation via the NO A (2)Sigma(+) state followed by photoexcitation of the Rydberg state that undergoes autoionization. The optical transition intensities from NO A state to nf Rydberg states were calculated, and the results agree closely with experiment. These results combined with circular dichroism measurements allow us to assign rotational quantum numbers to the nf Rydberg states even in a spectrum of relatively low resolution. We report the positions of these nf (upsilon,N,N(c)) Rydberg levels converging to the NO X (1)Sigma(+) upsilon(+) = 1 and 2 ionization limits where N is the total angular momentum excluding electron and nuclear spin and N(c) represents the rotational quantum number of the ion core. Our two-color optical-optical double resonance measurements cover the range of N from 15 to 28, N(c) from 14 to 29, and the principal quantum number n from 9 to 21. The electrostatic interaction between the Rydberg electron and the ion core is used to account for the rotational fine structure and a corresponding model is used to fit the energy levels to obtain the quadrupole moment and polarizability of the NO(+) core. Comparison with a multichannel quantum defect theory fit to the same data confirms that the model we use for the electrostatic interaction between the nf Rydberg electron and the ion core of NO well describes the rotational fine structure.     

Reaction dynamics of Y + O2--> YO(X,A',A)+ O(3P(J)) studied by the crossed-beam technique

The dynamics of the reaction, Y + O2--> YO + O was studied by using the crossed-beam technique at several collision energies from 10.3 to 52.0 kJ mol(-1). The Y atomic beam was generated by laser vaporization and crossed with the O2 beam at a right angle. Among the energetically accessible electronic states of YO, the formation of the A2Pi and A'2Delta states was observed by their chemiluminescence at all collision energies. By analyzing the chemiluminescence spectra of YO(A2Pi(1/2,3/2)-X2Sigma+), vibrational state distributions and relative populations of spin-orbit states were determined for YO(A2Pi(1/2,3/2)). At low collision energies, the vibrational distributions agree quite well with those expected from the statistical energy partitioning, while a little deviation from the statistical expectation was observed at the highest energy, 52.0 kJ mol(-1). The populations of two spin-orbit states are in good agreement with the statistical expectations at all collision energies. The vacuum ultraviolet laser-induced fluorescence (VUV-LIF) technique was employed to determine the distributions of spin-orbit states of the product O(3P(J)) at two collision energies, 20.7 and 52.0 kJ mol(-1). The line shapes of the O atom transitions were analyzed to determine relative branching ratio of the ground state to the excited states of YO, i.e. YO(X2Sigma+)+ O(3P(J))vs. YO(A2Pi and A'2Delta)+ O(3P(J)). The results showed that the electronically excited YO was formed with comparable amount with the ground state which is statistically more favorable, and suggested the occurrence of two mechanisms taking place in the title reaction.     

Laser induced amplified spontaneous emission from the B2Pi, L2Pi, and I2Sigma+ valence states of NO

Amplified spontaneous emission (ASE) from single rovibrational levels of valence (non-Rydberg) states of NO molecules has been investigated. The B2Pi (v=24 and 25), L2Pi (v=5 and 6), and I2Sigma+ (v=6) levels have been populated through laser optical-optical double resonance excitation via the Rydberg A2Sigma+ state. Term values for the 2Pi states have been determined with an accuracy of +/-0.03 cm(-1). Analyses of rotationally resolved dispersed ASE spectra in the near infrared region have shown that all the lower states belonged to the Rydberg states. The valence approximately Rydberg coupling in the upper manifolds has driven ASE systems from the valence to the Rydberg levels where they benefit from the strong intensities of inter-Rydberg transitions with Deltav=0. The experimentally predicted valence approximately Rydberg interactions have been compared with theoretical treatments.     

Study of ultrafast dynamics of 2-picoline by time-resolved photoelectron imaging

The dynamics of electronically excited states in 2-picoline is studied using femtosecond time-resolved photoelectron imaging spectroscopy. The internal conversion from the S(2) state to the vibrationally excited S(1) state is observed in real time. The secondarily populated high vibronic S(1) state deactivates further to the S(0) state. Photoelectron energy and angular distributions reveal the feature of ionization from the singlet 3p Rydberg states. In addition, variation of time-dependent anisotropy parameters indicates the rotational coherence of the molecule.     

Crossed molecular beam studies on the reaction dynamics of O(1D)+N2O

The reaction of oxygen atom in its first singlet excited state with nitrous oxide was investigated under the crossed molecular beam condition. This reaction has two major product channels, NO+NO and N2+O2. The product translational energy distributions and angular distributions of both channels were determined. Using oxygen-18 isotope labeled O(1D) reactant, the newly formed NO can be distinguished from the remaining NO that was contained in the reactant N2O. Both channels have asymmetric and forward-biased angular distributions, suggesting that there is no long-lived collision complex with lifetime longer than its rotational period. The translational energy release of the N2+O2 channel (fT = 0.57) is much higher than that of the NO+NO channel (fT = 0.31). The product energy partitioning into translational, rotational, and vibrational degrees of freedom is discussed to learn more about the reaction mechanism. The branching ratio between the two product channels was estimated. The 46N2O product of the isotope exchange channel, 18O+44N2O-->16O+46N2O, was below the detection limit and therefore, the upper limit of its yield was estimated to be 0.8%.     

Photodissociation channels for N2O near 130 nm studied by product imaging

The photodissociation of N(2)O at wavelengths near 130 nm has been investigated by velocity-mapped product imaging. In all, five dissociation channels have been detected, leading to the following products: O((1)S)+N(2)(X (1)Sigma), N((2)D)+NO(X (2)Pi), N((2)P)+NO(X (2)Pi), O((3)P) + N(2)(A (3)Sigma(+) (u)), and O((3)P) + N(2)(B (3)Pi(g)). The most significant channel is to the products O((1)S) + N(2)(X(1)Sigma), with strong vibrational excitation in the N(2). The O((3)P) + N(2)(A,B):N((2)D,(2)P) + NO branching ratio is measured to be 1.4 +/- 0.5, while the N(2)(A) + O((3)P(J)):N(2)(B) + O((3)P(J)) branching ratio is determined to be 0.84+/-0.09. The spin-orbit distributions for the O((3)P(J)), N((2)P(J)), and N((2)D(J)) products were also determined. The angular distributions of the products are in qualitative agreement with excitation to the N(2)O(D (1)Sigma(+)) state, with participation as well by the (3)Pi(v) state.     

Insight into the Rydberg states of CH

Ab initio electronic structure calculations of a relatively large number of Rydberg states of the CH radical were carried out employing the multireference single and double excitation configuration interaction (MRD-CI) method. A Gaussian basis set of cc-pV5Z quality augmented with 12 diffuse functions was used together with an extensive treatment of electron correlation. The main focus of this contribution is to investigate the 3d Rydberg complex assigned by Watson [Astrophys. J. 555, 472 (2001)] to three unidentified interstellar bands. The authors' calculations reproduce quite well the absolute excitation energies of the three components of the 3d complex, i.e., 2Sigma+(3dsigma), 2Pi(3dpi), and 2Delta(3ddelta), but not the energy ordering inferred from a rotational assignment of the 3d<--X 2Pi laboratory spectrum. The computation of the 4d complex is reported for the first time along with a number of other higher lying Rydberg species with an X 1Sigma+ core. The lowest Rydberg states belonging to series converging to the a 3Pi and A 1Pi excited states of CH+ are also calculated.     

Demonstration of the combination of slice imaging and Rydberg tagging for studies of photodissociation dynamics

The slice-imaging variant of photofragment ion imaging is combined with Rydberg tagging. The photodissociation of NO(2) at 355 nm is used as the test system and the NO fragments are Rydberg tagged by two-photon two-color excitation via the intermediate A (2)Sigma(+) state. Images obtained by this method are compared with ion images obtained in the same apparatus using the approach of Kitsopoulos and co-workers [Rev. Sci. Instrum. 72, 3848 (2001)]. Comparable resolution and angular distributions are obtained in the two cases. It is proposed that the method demonstrated here could provide a complementary approach to existing ion-imaging methods, especially where resonantly enhanced multiphoton ionization detection of fragments is problematic.     

Rotational state specific dissociation dynamics of HOD → H + OD via two-photon excitation to the C̃ electronic state

The dissociation dynamics of HOD via two-photon excitation to the C? state have been investigated using the H-atom Rydberg tagging time-of-flight (TOF) technique. The H-atom action spectrum for the C? ← X? transition shows resolved rotational structure. Product translational energy distributions and angular distributions have also been recorded for the H + OD channel for three excited levels each with k(a)′ = 2. From these distributions, quantum state distributions and angular anisotropy parameters (β2 and β4) for the OD product were determined. These results are consistent with the nonadiabatic predissociation picture illustrated in the one-photon dissociation process for H2O. The heterogeneous dissociation pathway via Coriolis coupling is the dominant dissociation process in the present study. A high proportion of the total available energy is deposited into the rotational energy of the OD product. The anisotropic recoil distributions reveal the distinctive contributions from the alignment of the excited states and the dissociation process. Comparisons are also made between the results for HOD and H2O via the equivalent rotational transitions. The OH bond energy, D(o)(H?OD), of the HOD molecule is also determined to be 41283.0 ± 5 cm(-1).