A multiphoton ionization study of the photodissociation dynamics of the S2 state of CH3ONO (original) (raw)
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The Journal of Chemical Physics, 1992
The photoionization process NO A 2~+ (v=O, N=22)-.NO+ X I~+ (v+=O, N+) +eis stu~ied with sufficient photoelectron energy resolution that the photoelectron angular distributions (PADs) associated with individual rotational levels N+ of the ion are determined. By. ionizing with left and right circularly polarized light and observing the change in the ro-tatIOnally resolved PADs, we can deduce all dynamical information, including the signs of the relative phase shifts of the photoelectron partial waves. This information constitutes the first complete description of the photoionization of a molecule. We discuss the consistency of our dynamical parameters with the Rydberg series of NO. We present a general formalism for (1 + 1') resonance-enhanced multiphoton ionization (REMPI) PADs for rotationally res~lve? i~n states using linearly po.larized light for excitation and elliptically polarized light for IOnIZatIOn. Based on the dynamIcal parameters determined from our fit, we use this formalism to predict the total system state, i.e., three-dimensional PADs and polarization of ion rotational levels following photoionization.
The Journal of Physical Chemistry A, 2011
The photodissociation of NO 3 has been studied using velocity map ion imaging. Measurements of the NO 2 þ O channel reveal statistical branching ratios of the O(3 P J) finestructure 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 þ O 2 products. The dominant pathway (>70% yield) is characterized by vibrationally excited O 2 (3 Σ g À , v = 5À10) and rotationally cold NO(2 Π), while the second pathway is characterized by O 2 (3 Σ g À , v = 0À4) 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 threecenter transition state, theoretical confirmation is needed.
The Journal of Chemical Physics, 1999
Strong orbital alignment is observed in the ground-state oxygen atom following photodissociation of NO 2 at 212.8 nm using ion imaging. The imaging method allows for investigation of the angular distribution of this alignment, providing insight into the dynamics in the frame of the molecule. The results are analyzed using a rigorous quantum mechanical theory yielding alignment parameters having direct physical significance. This alignment is dominated by a strong incoherent parallel contribution. In addition, the results reveal direct evidence of coherence between parallel and perpendicular contributions to the excitation of a polyatomic molecule, showing that the electron cloud in the recoiling atom ''remembers'' the original molecular plane.
The Journal of Chemical Physics, 1991
Time-of-flight photoelectron spectroscopy has been used to record energy-resolved photoelectron angular distributions (PADS) following (1 + 1') resonance-enhanced multiphoton ionization (REMPI) of NO via the ui = l,N, = 22 rovibrational level of the A 28 + state. The PADS corresponding to single rotational states of the resulting molecular ion show a strong dependence on the change in ion core rotation AN(-N +-Ni) and also on the angle between the linear polarization vectors of the two light beams. Broken reflection symmetry [I(19,$) #I(-0,#) ] is observed when the polarization vectors of the two light beams form an angle of 54.P. A fit to the PADS provides a complete description of this molecular photoionization, namely, the magnitudes and phases of the radial dipole matrix elements that connect the intermediate state to the ]lil) photoelectron partial waves (Refs. 1 and 2). This information is then used to predict unobserved quantities, such as ion angular momentum alignment and the full three-dimensional form of the PADS.
The Journal of Chemical Physics, 1999
Photoinitiated unimolecular decomposition rate constants of rotationally excited NO 2 molecules have been measured near dissociation threshold (D 0) by employing a double resonance technique. Rotational selectivity has been achieved by using narrow-linewidth ͑0.015 cm Ϫ1 ͒ infrared excitation to prepare specific rotational levels (NЈϭ1,3,...,15, K a Јϭ0) of the ͑1,0,1͒ vibrational level. The picosecond-resolution pump-probe technique has then been used to photodissociate the molecules thus tagged and to monitor the appearance of the NO product. Data have been obtained for two progressions of average excess energies, ͗E͘ϪD 0 : ͑i͒ 10 cm Ϫ1 ϩE 101 rot and ͑ii͒ 75 cm Ϫ1 ϩE 101 rot , where ͗E͘ denotes an average over the pump laser linewidth and E 101 rot is the rotational energy of the ͑1,0,1͒ X 2 A 1 intermediate vibrational level. The measured rate constants do not display any noticeable dependence on NЈ, which is a reflection of significant rovibronic interaction. Spinrotation interaction, which has been implicated as the main source of rovibronic coupling for small values of NЈ, is not likely to yield such a result. A model is proposed to describe the influence of rotation on the dissociation rate. The experimental data are consistent with a Coriolis coupling mechanism causing transitions to occur between K a levels.
Journal of Electron Spectroscopy and Related Phenomena, 2010
Low-energy photoelectron-vacuum ultraviolet (VUV) photon coincidences have been measured using synchrotron radiation excitation in the inner-valence región of the nitric oxide molecule. The capabilities of the coincidence setup were demonstrated by detecting the 2s _1-> 2p _1 radiative transitions in coincidence with the 2s photoelectron emission in Ne. In NO, the observed coincidence events are attributed to dissociative photoionization with excitation, whereby photoelectron emission is followed by fragmentation of excited NO + ions into 0 + +N* or N + + O* and VUV emission from an excited neutral fragment. The highest coincidence rate occurs with the opening of ionization channels which are due to correlation satellites of the 3a photoionization. The decay time of VUV photon emission was also measured, implying that specific excited states of N atoms contribute significantly to observed VUV emission.
Femtosecond Time-Resolved Photoelectron Angular Distributions Probed during Photodissociation of NO2
Physical Review Letters, 2000
Femtosecond time-resolved photoelectron angular distributions (PADs) are measured for the first time in the molecular frame of a dissociating molecule. Various stages of the dissociation process, NO 2 ! NO͑C 2 P͒ 1 O͑ 3 P͒, are probed using ionization of the NO͑C 2 P͒ fragment to NO 1 ͑X 1 S 1 ͒. The PADs evolve from forward-backward asymmetric with respect to the dissociation axis at short time delays (#500 fs) to symmetric at long time delays ($1 ps). Changes in the PADs directly reflect the timedependent separation and reorientation of the dissociating photofragments. PACS numbers: 33.80.Eh, 33.80.Gj, 33.80.Rv Measurements of photoelectron angular distributions (PADs) can provide a wealth of information on the nature of the molecular orbital that is ionized, the geometry and orientation of the molecule as the electron departs, and the dynamics of the photoionization process. Unfortunately, much of this information is lost when the PAD is measured from a randomly oriented sample of molecules. As a result, methods have been developed to measure PADs relative to the framework of the molecule. Recently, measurements of these molecular frame PADs have been used to probe shape resonances [1], inner shell ionization [2], valence shell ionization , and double ionization . Theoretical studies have shown that changes in PADs as a function of time, using femtosecond pumpprobe ionization, can provide a sensitive probe of molecular motions and orientations. In this paper, we present the first measurements of femtosecond time-resolved PADs in the molecular frame of a dissociating molecule. The changes observed in the PADs during the course of the dissociation directly reflect the time-dependent separation and reorientation of the fragments.
Photodissociation dynamics of NO2 at 248 nm
Journal of Photochemistry and Photobiology A: Chemistry, 1991
The dynamics of the photodissociation of NO2 at the excimer wavelength of 248 nm were investigated using a two-laser pump-probe technique. The unrelaxed NO photofragment was probed in vibrational levels O-9 by (1 + 1) resonantly enhanced multiphoton ionization (REMPI), using the y bands of NO in the A*X' +X*II transition. From the spectra, the relative populations of the ro-vibrational states of NO were determined. The resulting vibrational distribution was broad and inverted, peaking in v=5, and also exhibiting a large population in v = 0. The rotational distributions were non-thermal; a clearly bimodal distribution was observed in v =2. The spin states of NO were equally populated, but a preference was observed for the II over the II A doublet state. The dynamics of the process, dominated by the deposition of vibrational energy in the NO photofragment, could be qualitatively fitted by a Franck-Condon model, the shape of the vibrational distribution being attributed to the overlap of the vibrational wavefunction of NO in NO*' with that of the NO product.
The Journal of Chemical Physics, 1993
The direct photoionization process NOA 2~+ (v::::::O, N=13) ..... NO+ X I~+ (v+=O, N+) +eis studied by energy-and angle-resolved photoelectron spectroscopy by employing two-color resonance-enhanced multiphoton ionization (REMPI) via excitation of the NO A-X(O-O) R21 (11.5) transition. The photoelectron angular distributions (PADs) associated with individual rotational levels N+ of the ion are determined. Combined analysis of the newly obtained PADs and those reported earlier for the processes NO A 2~+ (v=O, N>20) ..... NO+ X I~+ (v+ =0, N+) +e-Vi~(P2i+Ql branch excitation shows that the photoionization dynamics is independent of the rotational quantum number of state to be ionized and of the spin state of the photoelectron. Quantitative comparison of our results with threshold photoelectron measurements provides strong evidence that ionization in the pulsed-electric-field threshold technique is not via direct p h o t o i o n i z a t i o n .- .
The Journal of Chemical Physics, 1997
The dissociation of nitric acid upon nb,O →* NO 2 excitation at 193 nm has been studied in a crossed laser-molecular beam apparatus. The primary reaction channels are OHϩNO 2 and OϩHONO. We measure the branching ratio between these two competing processes and determine ͑OHϩNO 2 ͒/͑OϩHONO͒ϭ0.50Ϯ0.05. Our experiments provide evidence of a minor OϩHONO pathway, which we assign to O( 3 P) and HONO in its lowest triplet state. The dominant pathway correlates to O( 1 D)ϩHONO(X 1 AЈ). The translational energy distributions reveal two distinct pathways for the OHϩNO 2 channel. One pathway produces stable NO 2 fragments in the 1 2 B 2 electronic state. The second pathway produces unstable NO 2 fragments which undergo secondary dissociation to NOϩO. We examine the influence of nonadiabaticity along the OHϩNO 2 reaction coordinate in order to explain the significant branching to this other channel. Finally, we introduce a new method for generating correlation diagrams for systems with electronic transitions localized on one moiety, in which we restrict the changes allowed in remote molecular orbitals along the reaction coordinate. Analysis of previously measured XϩNO 2 photofragment pathways in nitromethane and methyl nitrate provides further support for using a restricted correlation diagram to predict the adiabatic and nonadiabatic product channels.