Multiphoton Ionization Signal Research Articles

Singlet Molecular Oxygen Formation by O-Atom Recombination on Fused-Quartz Surfaces

Experimental results demonstrate () formation by catalytic O-atom surface recombination in a room-temperature fused-quartz flow-tube reactor. Resonance-enhanced multiphoton ionization is used to detect () downstream of a nitrogen discharge flow titrated with nitric oxide to introduce oxygen atoms. A calibration procedure based on ozone photodissociation is developed to quantify () resonance-enhanced multiphoton ionization signals. Partial pressures of () in the range of 2.9 to are measured in the ionization cell for O-atom partial pressures of 1.4–2.9 mtorr atomic oxygen introduced at the titration port. () could not be detected; an upper limit for the () partial pressure is one-fifth of the () partial pressure. A simple chemical kinetics model demonstrates that measured pressures cannot be explained by gas-phase chemistry alone and must involve O atoms participating in surface reactions. It is found that collisional deactivation of on the tube walls must be included to satisfactorily model the experimentally observed pressures and trends. Modeling results also suggest that the surface production yield is 10% or more.

Communication: A new spectroscopic window on hydroxyl radicals using UV + VUV resonant ionization

A 1 + 1' multiphoton ionization (MPI) detection scheme for OH radicals is presented. The spectroscopic approach combines initial excitation on the well-characterized A(2)Σ(+)-X(2)Π band system with vacuum ultraviolet (VUV) ionization via autoionizing Rydberg states that converge on the OH(+) A(3)Π ion state. Jet-cooled MPI spectra on the (1,0) and (2,0) bands show anomalous rotational line intensities, while initial excitation on the (0,0) band does not lead to detectable OH(+) ions. The onset of ionization with the (1,0) band is attributed to an energetic threshold; the combined UV + VUV photon energies are above the first member of the autoionizing (A(3)Π)nd Rydberg series. Comparison of the OH 1 + 1' MPI signal with that from single photon VUV ionization of NO indicates that the cross section for photoionization from OH A(2)Σ(+), v' = 1 is on the order of 10(-17) cm(2).

Multiphoton ionization of H+2 at critical internuclear separations: non-Hermitian Floquet analysis

We present ab initio time-dependent non-Hermitian Floquet calculations of multiphoton ionization (MPI) rates of hydrogen molecular ions subject to an intense linearly polarized monochromatic laser field with a wavelength of 800 nm. The orientation of the molecular axis is parallel to the polarization vector of the laser field. The MPI rates are computed for a wide range of internuclear separations R with high resolution in R and reproduce resonance and near-threshold structures. We show that enhancement of ionization at critical internuclear separations is related to resonance series with higher electronic states. The effect of two-centre interference on the MPI signal is discussed.

Photodissociation of N2O4 Adsorbed on Amorphous and Crystalline Water−Ice Films

The photodissociation dynamics of N2O4 adsorbed on amorphous and polycrystalline water−ice films has been studied at 193, 248, and 351 nm. The adsorbed states of N2O4 on the ice films at 80−140 K were investigated by infrared-absorption and temperature-programmed desorption measurements. Time-of-flight (TOF) spectra of the photofragments were measured by probing the resonance-enhanced multiphoton ionization signals of NO(v,J) and O(3Pj). Internally and translationally relaxed NO photofragments were observed. Their energy distributions were mostly independent of the states of the ice films and the photolysis laser wavelengths. The TOF distributions of NO(v = 0) consist of fast and slow components. The slow component is dominant. A large difference in the dissociation yields between amorphous and crystalline ice experiments was observed.

Detector based on time-resolved ion-induced voltage in laser multiphoton ionization and laser-induced fluorescence

The time-resolved ion-induced voltages obtained from multiphoton ionization (MPI) in gases were applied, for the first time, together with time-resolved laser-induced fluorescence (TR-LIF) for fast detection of organic compounds in ambient air. The target compounds were polycyclic aromatic hydrocarbons (PAHs) and nitroaromatics. The concepts of the proposed method are presented and examined. It is shown that the MPI signals allow for estimation of the molecular weight, while the fluorescence data supply additional quantitative and qualitative information. A 3D data presentation is suggested for obtaining selectivity. The proposed dimensions are the MPI-based mass estimation, the maximum LIF wavelength and the measured fluorescence lifetime. Since all required data are acquired simultaneously and since the setup can be continuously operated, this analytical concept is suggested for on-line applications. The common air components are not ionized at the selected wavelengths and only the organic pollutants are detected under these conditions. Moreover, it is shown that the resonant MPI mode provides a certain level of selectivity. Therefore, this setup may become the basis of an air sniffer.

Dips and peaks in four-photon resonant two-color multiphoton ionization signals

The dips and peaks observed in experiments over a broad hump of ionization signal in the four-photon resonant system are shown to be conviently discussed in terms of the real and virtual dressed state of the system. The real and the virtual dressed states differ in their dispersive response to the third harmonic wave generated in the medium. The dispersive and the absorptive response of the virtual and real dressed states coupled with the pressure of the gas are responsible for the dips and peaks and also the Fano-line like profile in the ionization signal in four-photon resonant multiphoton ionization experiments.

Sticking coefficient of the SiH2 free radical on a hydrogenated silicon-carbon surface

The sticking coefficient of SiH2 on a hydrogenated silicon-carbon surface is measured in a low-pressure pulsed-photolysis experiment. Thermal and vibrationally excited SiH2 are created by infrared multiphoton decomposition of n-butylsilane. The first-order wall loss rates of the radicals are determined from the time dependence of the resonance-enhanced multiphoton ionization signal. The sticking coefficients of SiH2 (∼0.1) and vibrationally hot SiH2 (>0.5) are determined from the measured first-order loss rate constants and the calculated wall collision rate constant.

Technique to study Rydberg states by multiphoton ionization spectroscopy

A new technique to study the Rydberg states of the barium atom has been developed. In this technique a multiphoton ionization signal is detected by selective excitation of the ground-state ion (6s) to an excited state (6p), which results in a collimated amplified spontaneous emission signal at the 6p → 5d transition of Ba+. Discrete Rydberg states, 6snl (l = 0, 2), as well as autoionizing Rydberg states, 5dnl (l = 0, 2) and 6pnl (l = 0, 2) are observed by this novel but simple method.

A NEW TECHNIQUE TO STUDY RYDBERG STATES BY MULTIPHOTON IONIZATION SPECTROSCOPY

A new technique to study the Rydberg states of the Ba atom has been developed. In this technique a Multiphoton Ionization signal is detected by selective excitation of the ground state ion (6s) to an excited state (6p), which results in a collimated Amplified Spontaneous Emission (ASE) signal at the 6p→5d transition of Ba*. Discrete Rydberg states, 6snl (l=0,2), as well as autoionizing Rydberg states, 5dnl (l=0,2) and 6pnl (l=0,2) are observed by this novel but very simple method.

Collisional effects in organometallic multiphoton dissociation/ionization experiments: the modeling of Townsend discharges

The authors have measured the effects of added buffer gas on the ionization (MPI) signal of various arene chromium tricarbonyls and Cr(CO)/sub 6/. Using basic gaseous electronics theory and experimental results on ion multiplication behavior in pure buffer gases, they have found the simplest method for quantitative analysis of collisional effects in MPI systems. They have applied this discharge model to the multiphoton ionization signal of chromium compounds as a function of helium, argon, and xenon pressures at a number of wavelengths and ion collection plant voltages. Where simple ion multiplication behavior holds, the measured ion signal is directly proportional to the initial MPI yield. This is of crucial importance for MPI studies where product branching ratios are being determined. When multiplication theory does not model the ion signal pressure dependence, other processes which may influence ion production in the system must be considered.

Optical–optical double resonance and double resonance multiphoton ionization in 7Li2 produced by single cw dye laser excitation

A comparison of fluorescence detection of optical–optical double resonance transitions with ionization detection of double resonance multiphoton ionization signals was made for Li2 vapor. The spectra in both cases were excited by a single broadband (30 GHz) cw dye laser and exhibited identical features. These arise from ‘‘accidental’’ two-photon double resonance transitions via real intermediate states and consist of hundreds of distinct transitions in the region 565–645 nm studied in this work. Analysis and assignment reveal that multiphoton excitations within the singlet manifold of electronic states of Li2 together with intermediate state rotational relaxation can account for the spectra. A table of vibrational and rotational assignments of the more intense features is available which includes a comparison between calculated and observed intensities for the transitions.

Multiphoton ionization of atomic hydrogen in methane pyrolysis

Hydrogen atoms are detected in situ in atmospheric pressure CH/sub 4/ pyrolysis by three-photon ionization via two-photon resonance of the 2S arrow to the left 1S transition at 243 nm. An expected deuterium isotope shift of 22.4 cm/sup -1/ is observed during CD/sub 4/ pyrolysis. Similar resonant multiphoton ionization signals of H atoms and D atoms are also observed from thermal dissociation of H/sub 2/ and D/sub 2/, respectively. The ionization signals are converted into absolute concentration via the H/sub 2/ in equilibrium 2H equilibrium. The detection sensitivity is estimated to be approx. 10/sup -9/-10/sup -8/ mole fraction at 1-atm pressure.

The emission spectrum of AsH 2(Ã 2A 1 → X̃ 2B 1) via uv photolysis of AsH 3

The AsH 2(Ã 2A 1 → X̃ 2B 1) emission spectrum (402–650 nm) following ArF laser photolysis of AsH 3 is reported. Seven bands are assigned and the relation ν = 19928 + (868 v' 2-3 v' 2 2)-(987 v″ 2-6 v″ 2 2) is obtained. The AsH 2 (Ã 2A 1) radiative lifetime is 130 ± 20 ns. Emission spectra from nascent As and a multiphoton ionization signal were also obtained.

Multiphoton ionization of Na and K in a modest electric field.

Multiphoton ionization (MPI) via two-photon resonance with high-lying Rydberg levels is observed on Na and K atoms in modest (12--120 V/cm) dc electric fields. The field-induced MPI signals are manifested to such an extent that the nominally forbidden MPI via two-photon resonance with the np levels is induced at the same signal levels as or more intensely than the allowed MPI via the ns and nd Rydberg levels. The intensity inversion between the allowed and the forbidden MPI signals is observed on the K atoms in the wavelength region where the two-photon energy is very close to the field-ionization limit.

Near infrared and visible two-photon transitions in the optogalvanic spectrum of neon

Eighty-two two-photon transitions for neon have been observed in the visible and near-infrared spectral regions for use as wavelength calibration standards. The optogalvanic signals were obtained from a commercial neon-filled hollow-cathode lamp, which was irradiated with the Raman shifted output as well as the unshifted output of a rhodamine 6G or rhodamine 640 pulsed dye laser. This arrangement yielded both one-and two-color two-photon spectra. The dependence of the optogalvanic signal on the laser power revealed that multiphoton ionization was occurring. The utility of the multiphoton ionization signal also was investigated.

Deposition of chromium films by multiphoton dissociation of chromium hexacarbonyl

Focused, pulsed, tunable dye lasers have been used to deposit chromium films by multiphoton dissociation of Cr(CO)6. The multiphoton dissociation of Cr(CO)6 is strongly wavelength dependent. Large increases in deposition rate occur below 350 nm, where the threshold between initial 1 and 2 photon absorption to the lowest lying dissociative state are reached in Cr(CO)6. Lack of correlation of deposition rate with multiphoton ionization (MPI) resonances, and polarity of a biased electrode in the cell, suggests multiphoton absorption leads to decomposition to Cr atoms followed by ionization. Power dependence of the deposition rates and MPI signals indicate saturation in the focal region for wavelengths 280<λ<340, for pulse energies 0.02–0.2 mJ. Deposition on cell windows not in the focal region also appears saturated at the shorter wavelengths. Films deposited on silicon and quartz substrates appear uniform, with visible evidence of melting and regrowth in the center of the deposits due to heating by subsequent pulses. Isotropic deposition of Cr atoms on the substrate from the gas phase, away from the irradiated spot, was also observed. Auger analysis of films deposited in the focal region suggest that significant quantities of carbon are present in the film. Alloying of the Cr with the underlying Si by laser heating of the substrate is also suggested.

Third-harmonic generation and multiphoton ionization in rare gases

Resonantly enhanced multiphoton ionization (MPI) and third-harmonic generation (THG) have been concurrently studied in the rare gases, xenon, krypton, and argon. The disappearance with increasing pressure of the MPI signals resonant with the three-photon allowed $6s$, $5s$, and $4s$ states, respectively, of the rare gases is linked to the appearance of THG. Conversely, THG is shown to be partially quenched at certain wavelengths by one- and two-photon absorption leading to ionization.

Effects of collective emission on multiphoton excitation and ionization near a three-photon resonance

We show that collective emission strongly affects excitation and the enhancement of multiphoton ionization near resonances involving odd numbers of photons. Such effects lead to striking shifts and broadening of resonantly enhanced multiphoton ionization signals near the odd photon resonance, and to a strong decrease in the ionization with increasing pressure.

Resonantly Enhanced Multiphoton Ionization and Third-Harmonic Generation in Xenon Gas

Resonantly enhanced multiphoton ionization signals through the $6s[\frac{3}{2}]J=1$ state of Xe rapidly disappear at pressures above 0.30 Torr; concurrently, intense, vacuum-ultra-violet third-harmonic radiation is detected in the forward direction. Laser excitation spectra of the vacuum ultraviolet light exhibit increasing blue shifts and bandwidths with increasing pressure. The observed competition between ionization and third-harmonic generation is discussed in terms of a model involving fast collective emission.