Resonance-enhanced Multiphoton Ionization Technique Research Articles

Detecting localized trace species in air using radar resonance-enhanced multi-photon ionization

A microwave-scattering-based resonance-enhanced multi-photon ionization technique is used to detect molecular species such as NO, CO, Xe, and Ar in pure form, and for standoff detection of trace species in atmospheric pressure air. In this paper,the spectra, dynamics, and the detection limits of trace species in air are studied. We demonstrate 10 m scale standoff detection of NO, and show that the system has a linear response down to the parts in 10(9) NO levels in ambient air.

Photodissociation Dynamics of 2-Bromopropane Using Velocity Map Imaging Technique

Photodissociation dynamics of 2-bromopropane in the A band was investigated at several wavelengths between 232 and 267 nm using resonance-enhanced multiphoton ionization technique combined with velocity map ion-imaging detection. The ion images of Br ((2)P(3/2)) and Br* ((2)P(1/2)) were analyzed to yield corresponding total translational energy and angular distributions. The total translational energy distributions showed that the channel leading to Br carried more internal energy in the 2-C(3)H(7) moiety than the channel leading to Br*. The anisotropy parameters of beta (Br) were obtained to be between 0.68 and 1.49, and beta (Br*) between 0.73 and 1.96, indicating that the Br* product originates from direct excitation of the (3)Q(0) state and the (1)Q(1) --> (3)Q(0) nonadiabatic transition, and the Br product from direct excitation of the (1)Q(1) or (3)Q(1) state and the (3)Q(0) --> (1)Q(1) nonadiabatic transition. The curve crossing probabilities were determined to be increase with the wavelength. As compared with the case of CH(3)Br, the two heavier branched CH(3) groups significantly enhance the Br ((2)P(3/2)) production from nonadiabatic contribution. The curve crossing from the (3)Q(0) to the (1)Q(1) surface is much higher than that of the reverse from the (1)Q(1) to the (3)Q(0) surface, which may have resulted from the difference in shape between the potential energy surfaces of the (3)Q(0) and (1)Q(1) states. Finally, based on the experimental data, the partial absorption cross sections of the A band for the (3)Q(0), (3)Q(1), and (1)Q(1) states were extracted.

Photodissociation of cis-, trans-, and 1,1-Dichloroethylene in the Ultraviolet Range: Characterization of Cl(2PJ) Elimination

By using photofragment velocity imaging detection coupled with a (2 + 1) resonance-enhanced multiphoton ionization technique, the elimination channel of spin-orbit chlorine atoms in photodissociation of cis-, trans-, and 1,1-dichloroethylene at two photolysis wavelengths of 214.5 and 235 nm is investigated. Translational energy and angular distributions of Cl((2)P(J)) fragmentation are acquired. The Cl((2)P(J)) fragments are produced by two competing channels. The fast dissociation component with higher translational energy is characterized by a Gaussian distribution, resulting from a curve crossing of the initially excited (pi, pi*) state to nearby repulsive (pi, sigma*) and/or (n, sigma*). In contrast, the slow component with a lower translational energy is characterized by a Boltzmann distribution, which dissociates on the vibrationally hot ground state relaxed from the (pi, pi*) state via internal conversion. cis-C(2)H(2)Cl(2) is found to have a larger branching of Boltzmann component than the other two isomers. The fraction of available energy partitioning into translation increases along the trend of cis- < trans- < 1,1-C(2)H(2)Cl(2). This trend may be fitted by a rigid radical model and interpreted by means of a torque generated during the C-Cl bond cleavage. The anisotropy parameters are determined, and the transition dipole moments are expected to be essentially along the C horizontal lineC bond axis. The results are also predicted theoretically. The relative quantum yields of Cl((2)P(J)) have a similar value for the three isomers at the two photolysis wavelengths.

The Gas Phase Anisole Dimer: A Combined High-Resolution Spectroscopy and Computational Study of a Stacked Molecular System

The gas phase structures of anisole dimer in the ground and first singlet electronic excited states have been characterized by a combined experimental and computational study. The dimer, formed in a molecular beam, has been studied by resonance-enhanced multiphoton ionization and high-resolution laser-induced fluorescence techniques. The assignment of the rotational fine structure of the S(1) <-- S(0) electronic transition origin has provided important structural information on the parallel orientation of aromatic rings of anisole moieties. By comparison with the DFT/TD-DFT computational results, it has been possible to infer the detailed equilibrium structure of the complex. The analysis of the equilibrium structure and interaction energy confirms that the anisole dimer is stabilized by dispersive interaction in the gas phase. This is, to the best of our knowledge, the first detailed work (reporting both theoretical and high-resolution experimental data) on an isolated cluster in the pi-stacking configuration.

Photodissociation Dynamics of &lt;em&gt;o&lt;/em&gt;-Bromotoluene at 234 and 267 nm

The photodissociation of o-bromotoluene was studied at 234 and 267 nm using velocity map imaging combined with a resonance-enhanced multiphoton ionization (REMPI) technique. Translational energy distributions suggested that ground state Br (2P(subscript 3/2)) and spin-orbit excited state Br(superscript *) (2P(subscript 1/2)) fragments were all generated via two dissociation channels: a fast channel and a slow channel. The anisotropy parameters of the fast channels were determined to be 1.15 (Br) and 0.55 (Br(superscript *)) at 234 nm, 0.90 (Br) and 0.60 (Br(superscript *)) at 267 nm. The anisotropy parameters of the slow channels were 0.12 (Br) and 0.14 (Br(superscript *)) at 234 nm, 0.11 (Br) and 0.10 (Br(superscript *)) at 267 nm. The Br and Br(superscript *) fragments of the slow channel were less anisotropic than those of the fast channel. The total relative quantum yields of Br (Φ (Br)) were 0.67 at 234 nm and 0.70 at 267 nm. Ground state Br (2P(subscript 3/2)) was the main product from the photolysis of o-bromotoluene at 234 and 267 nm. We propose that the fast channel originates from excitation of bound excited singlet (π, π(superscript *)) states followed by predissociation along repulsive (n, σ(superscript *)) states. The anisotropy parameters of the slow channels were close to zero indicating a hot dissociation mechanism on a highly vibrational ground state followed the internal conversion of the excited singlet state.

Nonadiabatic Transition in the A-Band Photodissociation of Ethyl Iodide from 294 to 308 nm by Using Velocity Imaging Detection

Photodissociation dynamics of ethyl iodide in the A-band is investigated at wavelengths between 294 and 308 nm using resonance-enhanced multiphoton ionization technique combined with velocity imaging detection. The I/I* branching, translational energy disposals, anisotropy parameters, and curve crossing probabilities of the dissociation channels are determined. The I(5p(2)P(3/2))-product channel is found to have hotter internal states of C(2)H(5), and the I*(5p(2)P(1/2)) channel is accompanied by colder C(2)H(5). Anisotropy parameters (beta) for the I* channel remain at 2.0, indicating that the I* production should originate from the (3)Q(0) state. The beta values are from 1.6 to 1.9 in the I-product channel, which comprises two components of direct excitation of (3)Q(1) and nonadiabatic transition between the (3)Q(0) and (1)Q(1) states. The curve crossing probability rises rapidly around the conical intersection but remains almost constant after passing through the curve crossing. The (1)Q(1) and (3)Q(0) states in the exit region are thus expected to cross almost parallel along the dissociation coordinate. As compared to the case of CH(3)I, the nonadiabatic transition probabilities are slightly enhanced by an ethyl-substituted group.

Release of Oxygen Atoms and Nitric Oxide Molecules from the Ultraviolet Photodissociation of Nitrate Adsorbed on Water Ice Films at 100 K

Production of O((3)P(J), J = 2, 1, 0) atoms from the 295-320 nm photodissociation of NO(3)- adsorbed on water polycrystalline ice films at 100 K was directly confirmed using the resonance-enhanced multiphoton ionization technique. Detection of the O atom signals required an induction period after deposition of HNO3 onto the ice film held at 130 K due to the slow ionization rate of HNO(3) to H+ and NO(3)- with a rate constant of k = (5.3 +/- 0.2) x 10(-3)s(-1). Translational energy distributions of the O atoms were represented by a combination of two Maxwell-Boltzmann energy distributions with translational temperatures of 2000 and 100 K. Direct detection of NO from the secondary photodissociation process was also successful. On the atmospheric implications, the influence of the direct release of the oxygen atoms into the air from NO(3)- adsorbed on the natural snowpack was included in an atmospheric model calculation on the mixing ratios of ozone and nitric oxide at the South Pole, and the results compared favorably with the field data.

Productions of I, I*, and C2H5 in the A-band photodissociation of ethyl iodide in the wavelength range from 245to283nm by using ion-imaging detection

Photodissociation dynamics of ethyl iodide in the A band has been investigated at several wavelengths between 245 and 283 nm using resonance-enhanced multiphoton ionization technique combined with velocity map ion-imaging detection. The ion images of I, I(*), and C(2)H(5) fragments are analyzed to yield corresponding speed and angular distributions. Two photodissociation channels are found: I(5p (2)P(3/2))+C(2)H(5) (hotter internal states) and I(*)(5p (2)P(1/2))+C(2)H(5) (colder). In addition, a competitive ionization dissociation channel, C(2)H(5)I(+)+h nu-->C(2)H(5)+I(+), appears at the wavelengths <266 nm. The I/I(*) branching of the dissociation channels may be obtained directly from the C(2)H(5) (+) images, yielding the quantum yield of I(*) about 0.63-0.76, comparable to the case of CH(3)I. Anisotropy parameters (beta) determined for the I(*) channel remain at 1.9+/-0.1 over the wavelength range studied, indicating that the I(*) production should originate from the (3)Q(0) state. In contrast, the beta(I) values become smaller above 266 nm, comprising two components, direct excitation of (3)Q(1) and nonadiabatic transition between the (3)Q(0) and (1)Q(1) states. The curve crossing probabilities are determined to be 0.24-0.36, increasing with the wavelength. A heavier branched ethyl group does not significantly enhance the I(5p (2)P(3/2)) production from the nonadiabatic contribution, as compared to the case of CH(3)I.

Vibrational excitation of the ν2 mode of NH 3 by infrared free electron laser

Vibrational excitation process in NH 3 by a mid-infrared free electron laser at Tokyo University of Science (FEL-SUT) is reported. The population of the ν 2 = 1 and 2 vibrationally excited states was monitored by the (2 + 1) resonance-enhanced multi-photon ionization (REMPI) technique via the B ∼ 1 E ″ Rydberg state. Excitation efficiency of the ν 2 = 1 ← ν 2 = 0 transition was approximately 30%. The rotationally resolved pathway for the vibrational ladder climbing up to ν 2 = 2 was identified through the analyses of the REMPI spectra and the FEL excitation spectra.

Photodesorption of hydrogen molecules physisorbed on Ag: Isotope dependence of translational-energy distribution

We report findings of our investigation on the non-thermal photodesorption dynamics of H 2(D 2) initially physisorbed on Ag. We photodesorbed the H 2(D 2) using a 6.4 eV pump photon, and then state-selectively probed the desorbing H 2(D 2) using the resonance-enhanced multi-photon ionization technique. The corresponding time-of-flight (TOF) spectra measured for the initial rotational states J = 1 and J = 0 indicate no particular J dependence. The mean translational temperature was ∼8 K for both H 2 and D 2. Using a coherent DIET (desorption induced by electronic transitions) model, we were able to reproduce the TOF spectra, including the observed isotope dependence.

Comprehensive On-Line Characterization of Complex Gas Mixtures by Quasi-Simultaneous Resonance-Enhanced Multiphoton Ionization, Vacuum-UV Single-Photon Ionization, and Electron Impact Ionization in a Time-of-Flight Mass Spectrometer: Setup and Instrument Characterization

This paper reports on a newly developed mobile mass spectrometer for comprehensive on-line analysis of complex gas mixtures such as ambient air or industrial process gases. Three ionization methods, namely, the resonance-enhanced multiphoton ionization (REMPI), vacuum-ultraviolet single-photon ionization (SPI), and electron impact ionization (EI) are implemented in this instrument and can be operated (quasi-) simultaneously. By means of this setup, a wide range of compounds can be analyzed due to the unique ionization selectivitiy and sensitivity profiles provided by the different ionization techniques. The mass spectrometer is designed for field application even under severe conditions. The REMPI technique is suitable for the selective and soft ionization (without fragmentation) of aromatic compounds at trace level (ppbv/pptv). The also soft but less selective SPI technique with 118-nm vacuum-ultraviolet laser pulses is used as a second laser-based ionization method. Mass spectra obtained by this technique show profiles of most organic compounds (aliphatic and aromatic species) and of some low IP inorganic substances (e.g., ammonia, nitrogen oxide) down to ppbv concentrations. In addition to the laser-based ionization techniques, EI ionization can be used for analysis of the bulk components such as water, oxygen, nitrogen, and carbon dioxide as well as for detection of inorganic minor components such as HCN or HCl from combustion flue gases at ppmv concentration levels. Each method yields specific mass spectrometric information of the sample composition. Special techniques have been developed to combine the three ionization methods in a single mass spectrometer and to allow the quasi-parallel application of all three ionization techniques.

Photodissociation Dynamics of Dimethyl Sulfide Following Excitation within the First Absorption Band

The technique of resonance-enhanced multiphoton ionization with time-of-flight mass spectrometry (REMPI-TOF MS) has been used to study the photodissociation of fully deuterated dimethyl sulfide (CD3SCD3) following excitation at several wavelengths within the first absorption band (215−229 nm). Analysis of measured time-of-flight profiles of the nascent CD3 products indicates a strongly anisotropic photodissociation, with approximately 70−80% of the available energy appearing as fragment recoil translation. A hybrid statistical−impulsive dissociation model predicts a photolysis wavelength dependence of the translational energy release that compares favorably with experiment, supporting the suggestion (Manaa, M. R.; Yarkony, D. R. J. Am. Chem. Soc. 1994, 116, 11444) that both the 11A‘ ‘ and 21A‘ ‘ excited states play an important role in the photodissociation process. An analysis of nascent CD3 (μ, v, J) vector correlations, where the fragments are assumed to recoil axially along the CD3S−CD3 bond, adequate...

Rotational line strengths of the v2-active two-photon transitions of the methyl radical

To extract information on the rotational population distributions of the methyl radical from photodissociation by the 2+1 resonance-enhanced multiphoton ionization technique, its rotational line strength formulas of the two-photon transitions have been reexamined. Symmetry-adapted rovibronic-nuclear spin wave functions of CH3 and CD3 in the |X̃ 2A2″〉 and |np 2A2″〉 electronic states were utilized in the derivation. Transformation properties of the rovibronic and nuclear spin basis functions under the permutation-inversion group D3h(M) have been employed to construct the total wave functions which follow the appropriate statistics of CH3 and CD3, respectively. Explicit expressions of the two-photon rotational line strengths of the v2-active vibronic bands of the methyl radical were reported.

Photodissociation of Chlorine Molecules Adsorbed on Amorphous and Crystalline Water Ice Films

The photodissociation dynamics of chlorine molecules adsorbed on amorphous and crystalline water ice films was investigated at 351 nm. The ice films were prepared on a gold polycrystalline substrate at 80−140 K. Time-of-flight spectra of the photofragment chlorine atoms, measured with the resonance-enhanced multiphoton ionization technique, were simulated with a composite of two translational energy distributions: a Gaussian distribution and a flux-weighted Maxwell−Boltzmann distribution. For both amorphous and crystalline ice films, the Gaussian distribution is characterized by the average energy 〈Et〉 = 0.38 ± 0.02 eV, while the Maxwell−Boltzmann one by 〈Et〉 = 0.12 ± 0.01 eV. The Gaussian distribution is attributable to the chlorine atoms produced from the direct photodissociation of Cl2, while the Maxwell−Boltzmann characterizes those having undergone strong relaxation processes. The observed translational energy distributions for amorphous and crystalline ice films were similar to each other, but the ...

Ionization and dissociation mechanisms of ketene using resonance-enhanced multiphoton ionization mass spectrometer: (2+2) versus (2+1) schemes

By using a resonance-enhanced multiphoton ionization (REMPI) technique, we have studied ionization and photodissociation mechanisms of ketene. Prior to ionization, the jet-cooled ketene is first excited at a wavelength 355 nm to a 3py,00 Rydberg state through a two-photon absorption. The (2+2) and (2+1) REMPI schemes may be distinguished depending on the impinging laser energy. If the (2+2) REMPI process dominates, the ketene ion is produced by the autoionization of a superexcited state, which lies in a rovibrationally excited Rydberg state. The autoionization then occurs due to energy transfer from nuclear to electronic degrees of freedom. The CH2+ is fragmented following two schemes. One is a consecutive process, i.e., the fragment ion is produced from the autoionized ketene. This conclusion is supported by a series of measurements of pulse field and pressure effects in this work. The factors of pulse field and collisions may enhance the autoionization rate significantly, imposing the same influence on the ketene ion and CH2+. The second is a dissociative ionization, i.e., the CH2+ ion is fragmented from the superexcited state in competition with the autoionization, as reported previously. These two schemes exclude the probability of ionic ladder or neutral ladder dissociation mechanisms. On the other hand, if the (2+1) REMPI scheme dominates under a low ionizing laser energy, the ketene ions are led by a direct photoionization. The increase of either pulse field intensity or interacting duration simply shortens the arrival time of the ketene ion on the detector. Finally, a time-resolved ketene ion spectrum is measured to characterize temporal behaviors for the autoionization and direct photodissociation. The relaxation lifetime for the autoionized ketene is found to be much faster than that for the directly photoionized source.

Vibronic spectra of the allyl radical at 6–8 eV with resonance-enhanced multiphoton ionization technique

The allyl radical was produced in molecular beam by pyrolysis of allyl iodide. The vibronic spectra from ground state to six new electronic states of the allyl radical at 6–8 eV, π →3dxz, π →3dxy and π →ns (n=4, 6, 7, 8) were observed firstly with the aid of time-of-flight mass spectros copy and resonance-enhanced multiphoton ionization technique. Vibrational progression ofv 7(C3 bend) with gross spacing of about 430 cm−1 was observed inns Rydberg states. The adiabatic ionization potential of the allyl radical was obtained to be (65641 ± 20) cm−1 ((8.138 ± 0.002) eV) by fitting the term values ofns (n=4,6,7,8) Rydberg states with Rydberg formula.

A mobile mass spectrometer for comprehensive on-line analysis of trace and bulk components of complex gas mixtures: parallel application of the laser-based ionization methods VUV single-photon ionization, resonant multiphoton ionization, and laser-induced electron impact ionization.

A newly developed compact and mobile time-of-flight mass spectrometer (TOFMS) for on-line analysis and monitoring of complex gas mixtures is presented. The instrument is designed for a (quasi-)simultaneous application of three ionization techniques that exhibit different ionization selectivities. The highly selective resonance-enhanced multiphoton ionization (REMPI) technique, using 266-nm UV laser pulses, is applied for selective and fragmentationless ionization of aromatic compounds at trace levels (parts-per-billion volume range). Mass spectra obtained using this technique show the chemical signature solely of monocyclic (benzene, phenols, etc.) and polycyclic (naphthalene, phenathrene, indol, etc.) aromatic species. Furthermore, the less selective but still fragmentationless single photon ionization (SPI) technique with 118-nm VUV laser pulses allows the ionization of compounds with an ionization potential below 10.5 eV. Mass spectra obtained using this technique show the profile of most organic compounds (aliphatic and aromatic species, like nonane, acetaldehyde, or pyrrol) and some inorganic compounds (e.g., ammonia, nitrogen monoxide). Finally, the nonselective ionization technique laser-induced electron-impact ionization (LEI) is applied. However, the sensitivity of the LEI technique is adjusted to be fairly low. Thus, the LEI signal in the mass spectra gives information on the inorganic bulk constituents of the sample (i.e., compounds such as water, oxygen, nitrogen, and carbon dioxide). Because the three ionization methods (REMPI, SPI, LEI) exhibit largely different ionization selectivities, the isolated application of each method alone solely provides specific mass spectrometric information about the sample composition. Special techniques have been developed and applied which allow the quasi-parallel use of all three ionization techniques for on-line monitoring purposes. Thus, a comprehensive characterization of complex samples is feasible jointly using the characteristic advantages of the three ionization techniques. Laboratory applications show results on rapid overview characterization of mineral oil-based fuels and coffee headspace. The first reported field applications include timely resolved on-line monitoring results on automobile exhausts and of waste incineration flue gas.

Vibronic spectra of the allyl radical at 6—8 eV with resonance-enhanced multiphoton ionization technique

The allyl radical was produced in molecular beam by pyrolysis of allyl iodide. The vibronic spectra from ground state to six new electronic states of the allyl radical at 6—8eV, π →3d xz , π →3d xy ,and π → ns ( n =4,6,7,8) were observed firstly with the aid of time-of-flight mass spectroscopy and resonance-enhanced multiphoton ionization technique. Vibrational progression of ν 7 (C 3 bend) with gross spacing of about 430 cm -1 was observed in ns Rydberg states. The adiabatic ionization potential of the allyl radical was obtained to be (65641 ± 20)cm -1 ((8.138 ± 0.002) eV) by fitting the term values of ns ( n =4,6,7,8)Rydberg states with Rydberg formula.

Velocity map imaging and REMPI study of the photodissociation of CH 3SCH 3 from the first absorption band

The photodissociation of dimethyl sulfide (CH 3SCH 3) at 229 nm has been studied employing a combination of velocity map imaging and time-of-flight resonance-enhanced multiphoton ionization techniques to detect the CH 3 products. Translational energy and recoil angle distributions as well as rotational state populations have been determined for the CH 3 photofragments formed in the ground vibrational state. The electronic excitation of CH 3SCH 3 to the first absorption band is found to produce fast CH 3S+CH 3 ( v=0) recoiling products with a negative spatial anisotropy parameter of β=−0.85±0.05. The CH 3 ( v=0) products are rotationally cold, the rotational distribution peaking at N′′=3–4.

Formula omitted] isotope effect in the UV photodissociation of N 2O

14 N 2 16 O , 14 N 15 N 16 O and 15 N 14 N 16 O were photolyzed between 202 and 206 nm. The photodissociation products, O( 1 D 2) and N 2( X 1 ∑ g +) , were monitored by a resonance-enhanced multiphoton ionization technique. The O( 1 D 2) signal was weaker when the heavy isotopomers were photolyzed at 205.47 nm. The isotope effect was as large as 10% for 14 N 15 N 16 O , while it was 3% for 15 N 14 N 16 O . The absorption coefficients must be smaller for the heavy isotopomers. An inverse isotope effect was observed when N 2( X 1 ∑ g +) was monitored via the a″ 1 ∑ g + state. 14 N 2 signal was less than that for 14 N 15 N . There must be an isotope effect in the detection efficiency of highly rotationally excited N 2( X 1 ∑ g +) .