Multiphoton Ionization Of NO Research Articles

Molecular beam scattering of NO+Ne: a joint theoretical and experimental study.

The collision dynamics of the NO+Ne system is investigated in a molecular beam scattering experiment at a collision energy of 1055 cm(-1). Employing resonance enhanced multiphoton ionization of NO, we measured state-resolved integral and differential cross sections for the excitation to various levels of both spin-orbit manifolds. The dependence of the scattered intensity on the laser polarization is used to extract differential quadrupole moments for the collision induced angular momentum alignment. The set of cross section data is compared with results of a full quantum mechanical close coupling calculation using the set of ab initio potential energy surfaces of Alexander et al. [J. Chem. Phys. 114, 5588 (2001)]. In previous work, it was found that the positions and rotational substructures for the lowest bend-stretch vibrational states derived from these surfaces agree very well with the observed spectrum of the NO-Ne complex. For the same potential, we find that the calculated cross sections show a less satisfactory agreement with the experimental data. While the overall Jf dependence and magnitude of the integral and differential cross sections are in good agreement, noticeable discrepancies exist for the angle dependence of the differential cross sections. In general, the calculated rotational rainbow structures are shifted towards larger scattering angles indicating that the anisotropy of the potential is overestimated in the fit to the ab initio points or in the ab initio calculation itself. For most states, we find the measured alignment moments to be in excellent agreement with the results of the calculation as well as with predictions of sudden models. Significant deviations from the sudden models are observed only for those fine-structure changing collisions which are dominated by forward scattering. Results of the full quantum calculation confirm the deviations for these states.

REMPI detection of alignment in NO collisions

Abstract 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.

Selecting ionization path by dynamic stark shift with strong laser pulse

Multiphoton ionization of NO via intermediate Rydberg states with ultra-short laser pulses is investigated with time-resolved photoelectron spectroscopy in combination with femtosecond pump–probe technology. The Rydberg states of NO, which are characterized by obvious ac-Stark shift in ultra-strong laser field, can be tuned in resonance to ionize NO molecule at one’s will with identical laser pulses, i.e., one can ‘select’ resonance path to ionization. The results shown in this Letter demonstrate that the states holding notable dynamic Stark shift provide us another dimension to chemical control with strong laser field.

A simple kinetic model of ion production and loss in the laser excitation of nitric oxide

A kinetic model is proposed to describe the behavior of molecular multiphoton ionization for nitric oxide. The dissociative attachment and several related radiationless processes are included in this model. It helps the understanding of the ion production and loss in the system of resonance enhanced multiphoton ionization of NO.

Rotationally resolved photoelectron spectra in (2 + 1) resonance-enhanced multiphoton ionization of NO via the C 2Π Rydberg state

Results of theoretical studies of rotational branching ratios and photoelectron angular distributions for (2 + 1) resonance-enhanced multiphonon ionization (REMPI) of NO via the Q 21(13.5) branch of the C 2Π(3pπ) Rydberg state are reported and compared with recent measurements. Both calculated and measured spectra reveal that the Δ N=±2 peaks (ee line), which are very weak for the laser polarization parallel to the direction of photoelectron detection, have about 1/4 the intensity of the main Δ N=0 peak when the laser polarizaiton is rotated by 90°. This dependence on laser polarization is due to strong inteference among partial wave components of the photoelectron matrix element. This behavior is very evident in the photoelectron angular distribution. In addition, a Cooper minimum is predicted in the d wave of the 3pπ→ Kδ ionization channel.

Effects of Cooper minima in resonance enhanced multiphoton ionization-photoelectron spectroscopy of NO via the D 2Σ+ and C 2Π Rydberg states

Cooper minima are predicted to occur in the 3pσ→kσ(l=2) and 3pσ→kπ(l=2) channels in the resonance enhanced multiphoton ionization of NO via the D 2Σ+(3pσ) Rydberg state. The low energy kσ(l=2) Cooper minimum leads to the observed ΔN=N+−N′=0 photoelectron peak, in addition to the ΔN=±1, ±3 peaks seen in the rotational spectra. The Cooper minima are accompanied by significant l mixing in the continuum due to the nonspherical molecular potential and result in a strong dependence of rotational branching ratios and angular distributions on photoelectron kinetic energy. A Cooper minimum is also predicted in the 3pπ→kδ(l=2) channel for photoionization of the C 2Π(3pπ) Rydberg state. The effect of this Cooper minimum on photoelectron spectra and photoelectron angular distributions is also investigated.

(2+1′) rotationally resolved resonance enhanced multiphoton ionization via the E 2∑+(4s,3d) and H 2∑+(3d,4s) Rydberg states of NO

The results of studies of ionic rotational branching ratios and photoelectron angular distributions resulting from (2+1′) resonance enhanced multiphoton ionization of NO via various high J (≊21.5) rotational branches of the E2∑+(4s,3d) and H2∑+(3d,4s) Rydberg states are presented. The rotational branching ratios show the expected ΔN=even rotational propensity rule with very small ΔN=odd signals. The branching ratios for the E2∑+ state are seen to be independent of photoelectron energy with the ΔN=+2 signals strongest and no appreciable higher rotational transfer peaks (‖ΔN‖≥3). The higher rotational transfer signal for ionization of the H2∑+ state are also negligible but the rotational branching ratios are strongly energy dependent due to a Cooper minimum in the l=3 partial wave of the kσ- and kπ-continua at a photoelectron kinetic energy of 2.6 eV and 2.9 eV, respectively. This leads to a strong rotational selectivity that can be exploited to produce ions in a specific rotational level. These consequences of Cooper minima close to threshold are quite general and their influence on rotational distributions should be readily observable in other molecular systems. The photoelectron angular distributions via both states show a strong energy dependence with a rapid change in the angular distributions around the Cooper minimum associated with the H2∑+ state.

Bandwidth and tuning range of an ArF laser measured by 1 + 1 resonantly enhanced multiphoton ionization of NO

The tuning range and bandwidth of an ArF laser were measured using 1 + 1 resonantly enhanced multiphoton ionization of NO. Operated as an injection-seeded oscillator/amplifier combination, the tuning range was 51,560-51,810 cm(-1); operated with single pass amplification of the oscillator, the tuning range was 51,560-51,765 cm(-1). In both cases, the laser bandwidth, determined from the linewidth, was 0.21 +/- 0.06 cm(-1). Rotational lines in the beta(7,0), gamma(3,O), and epsilon(0,1) bands were observed including several previously unreported lines.

Rotationally resolved photoelectron angular distributions in resonance enhanced multiphoton ionization of NO

We report calculated ionic rotational branching ratios and associated photoelectron angular distributions for (1+1′) resonance enhanced multiphoton ionization (REMPI) via the R21(20.5), P21+Q11(25.5), and P11(22.5) branches of the A 2 Σ+(3sσ) state of NO. The branching ratios are dominated by even angular momentum transfer peaks, in agreement with the ΔN+l=odd (ΔN≡N+−Ni ) selection rule. Whereas the calculated photoelectron angular distributions are very branch dependent alignment, the ionic branching ratios are found to be less so. The present calculated results agree well with the experimental results of Allendorf et al.

Ionic clusters grown from isolated ion seeds

Resonantly enhanced multi-photon ionization of NO produces cleanly NO + in a supersonic free jet expansion. Sampling of this expansion through a skimmer with a time-of-flight mass spectrometer reveals that cluster species containing as many as 15 NO molecules and 20 Ar atoms can be grown from the isolated NO + seed. For large clusters (NO) χ +, species with odd χ appear strongly favored.

Ionic rotational branching ratios in resonant enhanced multiphoton ionization of NO via the A 2Σ+(3sσ) and D 2Σ+(3pσ) states

We present the results of ab initio calculations of the ionic rotational branching ratios in NO for a (1+1) REMPI (resonant enhanced multiphoton ionization) via the A 2Σ+(3sσ) state and a (2+1) REMPI via the D 2Σ+(3pσ) state. Despite the atomic-like character of the bound 3sσ and 3pσ orbitals in these resonant states, the photoelectron continuum exhibits strong l mixing. The selection rule ΔN+l=odd (ΔN≡N+−Ni) implies that the peaks in the photoelectron spectrum corresponding to ΔN=odd (±1,±3) are sensitive to even partial waves while those corresponding to even ΔN probe the odd partial waves in the photoelectron continuum. Recent experimental high resolution photoelectron studies have shown a strong ΔN=0 peak for ionization via the A 2Σ+ and the D 2Σ+ states, indicating a dominance of odd-l partial waves. While this seems natural for ionization out of the 3sσ orbital, it is quite anomalous for 3pσ ionization. Based on extensive bound calculations, Viswanathan et al. [J. Phys. Chem. 90, 5078 (1986)] attribute this anomaly to a strong l mixing in the electronic continuum caused by the nonspherical molecular potential. We have performed ab initio calculations of the rotational branching ratios and compared them with the experimental results. The electronic continuum shows a significant p-wave component which leads to the large ΔN=0 peak in both cases. Calculations are performed for both rotationally ‘‘clean’’ and ‘‘mixed’’ branches. The relative heights of the peaks are very sensitive to the photoelectron kinetic energy for the D 2Σ+ state and less so for the A 2Σ+ state. This is a direct consequence of the l mixing in the continuum.

Rotational branching ratios in (1+1) resonant-enhanced multiphoton ionization of NO via the A 2 Sigma + state.

The rotational branching ratios resulting from (1+1) resonant-enhanced multiphoton ionization of NO via the A 2Σ+ Rydberg state are analyzed. Theoretical results using ab initio molecular parameters agree reasonably well with recent experimental data. More importantly, the analysis underscores the importance of the molecular nature of the problem and its resulting complexities. It is shown that, for photoionization of a Σ state that leaves the ion in a Σ state, the allowed rotational states of the ion satisfy the selection rule ΔN+l=odd, where ΔN is the difference in (electronic + rotational) quantum numbers for the neutral and for the ion, and l is the partial wave of the electron. Based on this selection rule, it follows that the predominantly gerade 3sσ Rydberg orbital of the A 2Σ+ state couples only to the ungerade channel in the continuum (l odd), thereby suppressing the ΔN=±1 peaks, in agreement with experiment. The molecular nature of the ionic potential leads to strong l mixing in electronic continuum orbitals. In fact, the influence of a nearby shape resonance causes the f wave to be dominant in the σ channel.