Resonance Enhanced Multiphoton Ionization Signal Research Articles

Photodissociation Dynamics of Vinoxy Radical via the B̃2A″ State: The H + CH2CO Product Channel.

Photodissociation dynamics of the jet-cooled vinoxy radical (CH2CHO) via the B̃2A″ state was studied in the near-ultraviolet (near-UV) region of 308-328 nm using high-n Rydberg H atom time-of-flight (HRTOF) and resonance-enhanced multiphoton ionization (REMPI) techniques. The vinoxy radical beam was produced by 193 nm photolysis of ethyl vinyl ether followed by supersonic expansion. The H + CH2CO product channel was observed directly in the H atom TOF spectra. The H atom photofragment yield (PFY) spectra were obtained by integrating the H atom TOF spectra and measuring the H atom REMPI signals, and showed several vibronic bands of the B̃2A″ state. The translational energy distributions of the H + CH2CO products, P(ET)'s, were obtained at several vibronic transitions. The P(ET) distributions were broad, peaking at a low energy of ∼3500 cm-1. The product translational energy release was moderate; the average translational energy release in the maximum available energy, ⟨fT⟩, was in the range of 0.24-0.27. The product angular distributions in this wavelength region were slightly anisotropic, with the β parameter in the range of 0.10-0.24. The near-UV photodissociation mechanism of the H + CH2CO product channel of the vinoxy radical is consistent with unimolecular dissociation on the electronic ground state (X̃2A″) following internal conversion from the B̃2A″ state to the Ã2A' state and then to the X̃2A″ state (although unimolecular dissociation from the first excited Ã2A' may also contribute).

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.

Coherent superposition of M-states in a single rovibrational level of H2 by Stark-induced adiabatic Raman passage

We prepare an ensemble of isolated rovibrationally excited (v = 1, J = 2) H2 molecules in a phase-locked superposition of magnetic sublevels M using Stark-induced adiabatic Raman passage with linearly polarized single-mode pump (at 532 nm, ∼6 ns pulse duration, 200 mJ/pulse) and Stokes (699 nm, ∼4 ns pulse duration, 20 mJ/pulse) laser excitation. A biaxial superposition state, given by [line]ψ(t)⟩ = 1/√(2)[[line]ν = 1, J = 2, M = -2⟩ - [line]ν = 1, J = 2, M = +2⟩], is prepared with linearly but cross-polarized pump and Stokes laser pulses copropagating along the quantization z-axis. The degree of phase coherence is measured by using the O(2) line of the H2 E,F-X (0,1) band via 2 + 1 resonance enhanced multiphoton ionization (REMPI) at 210.8 nm by recording interference fringes in the REMPI signal in a time-of-flight mass spectrometer as the direction of the UV laser polarization is rotated using a half-wave plate. Nearly 60% population transfer from H2 (v = 0, J = 0) ground state to the superposition state in H2 (v = 1, J = 2) is measured from the depletion of the Q(0) line of the E,F-X (0,0) band as the Stokes frequency is tuned across the (v = 0, J = 0) → (v = 1, J = 2) Raman resonance.

Communication: Transfer of more than half the population to a selected rovibrational state of H2 by Stark-induced adiabatic Raman passage

By using Stark-induced adiabatic Raman passage (SARP) with partially overlapping nanosecond pump (532 nm) and Stokes (683 nm) laser pulses, 73% ± 6% of the initial ground vibrational state population of H(2) (v = 0, J = 0) is transferred to the single vibrationally excited eigenstate (v = 1, J = 0). In contrast to other Stark chirped Raman adiabatic passage techniques, SARP transfers population from the initial ground state to a vibrationally excited target state of the ground electronic surface without using an intermediate vibronic resonance within an upper electronic state. Parallel linearly polarized, co-propagating pump and Stokes laser pulses of respective durations 6 ns and 4.5 ns, are combined with a relative delay of ~4 ns before orthogonally intersecting the molecular beam of H(2). The pump and Stokes laser pulses have fluences of ~10 J/mm(2) and ~1 J/mm(2), respectively. The intense pump pulse generates the necessary sweeping of the Raman resonance frequency by ac (second-order) Stark shifting the rovibrational levels. As the frequency of the v = 0 → v = 1 Raman transition is swept through resonance in the presence of the strong pump and the weaker delayed Stokes pulses, the population of (v = 0, J = 0) is coherently transferred via an adiabatic passage to (v = 1, J = 0). A quantitative measure of the population transferred to the target state is obtained from the depletion of the ground-state population using 2 + 1 resonance enhanced multiphoton ionization (REMPI) in a time-of-flight mass spectrometer. The depletion is measured by comparing the REMPI signal of (v = 0, J = 0) at Raman resonance with that obtained when the Stokes pulse is detuned from the Stark-shifted Raman resonance. No depletion is observed with either the pump or the Stokes pulses alone, confirming that the measured depletion is indeed caused by the SARP-induced population transfer from the ground to the target state and not by the loss of molecules from photoionization or photodissociation. The two-photon resonant UV pulse used for REMPI detection is delayed by 20 ns with respect to the pump pulse to avoid the ac Stark shift originating from the pump and Stokes laser pulses. This experiment demonstrates the feasibility of preparing a large ensemble of isolated molecules in a preselected single quantum state without requiring an intermediate vibronic resonance.

Photofragmentations, state interactions, and energetics of Rydberg and ion-pair states: Two-dimensional resonance enhanced multiphoton ionization of HBr via singlet-, triplet-, Ω = 0 and 2 states

Mass spectra were recorded for one-colour resonance enhanced multiphoton ionization (REMPI) of H(i)Br (i = 79, 81) for the two-photon resonance excitation region 79,040-80,300 cm(-1) to obtain two-dimensional REMPI data. The data were analysed in terms of rotational line positions, intensities, and line-widths. Quantitative analysis of the data relevant to near-resonance interactions between the F(1)Δ(2)(v' = 1) and V(1)Σ(+)(v' = m + 7) states gives interaction strengths, fractional state mixing, and parameters relevant to dissociation of the F state. Qualitative analysis further reveals the nature of state interactions between ion-pair states and the E(1)Σ(+) (v' = 1) and H(1)Σ(+)(v' = 0) Rydberg states in terms of relative strengths and J' dependences. Large variety in line-widths, depending on electronic states and J' quantum numbers, is indicative of number of different predissociation channels. The relationship between line-widths, line-shifts, and signal intensities reveals dissociation mechanisms involving ion-pair to Rydberg state interactions prior to direct or indirect predissociations of Rydberg states. Quantum interference effects are found to be important. Moreover, observed bromine atom (2 + 1) REMPI signals support the importance of Rydberg state predissociation channels. A band system, not previously observed in REMPI, was observed and assigned to the k(3)Π(0)(v' = 0) ←← X transition with band origin 80,038 cm(-1) and rotational parameter B(v('))=7.238 cm(-1).

Thermal Decomposition Mechanisms of the Methoxyphenols: Formation of Phenol, Cyclopentadienone, Vinylacetylene, and Acetylene

The pyrolyses of the guaiacols or methoxyphenols (o-, m-, and p-HOC(6)H(4)OCH(3)) have been studied using a heated SiC microtubular (μ-tubular) reactor. The decomposition products are detected by both photoionization time-of-flight mass spectroscopy (PIMS) and matrix isolation infrared spectroscopy (IR). Gas exiting the heated SiC μ-tubular reactor is subject to a free expansion after a residence time of approximately 50-100 μs. The PIMS reveals that, for all three guaiacols, the initial decomposition step is loss of methyl radical: HOC(6)H(4)OCH(3) → HOC(6)H(4)O + CH(3). Decarbonylation of the HOC(6)H(4)O radical produces the hydroxycyclopentadienyl radical, C(5)H(4)OH. As the temperature of the μ-tubular reactor is raised to 1275 K, the C(5)H(4)OH radical loses a H atom to produce cyclopentadienone, C(5)H(4)═O. Loss of CO from cyclopentadienone leads to the final products, acetylene and vinylacetylene: C(5)H(4)═O → [CO + 2 HC≡CH] or [CO + HC≡C-CH═CH(2)]. The formation of C(5)H(4)═O, HCCH, and CH(2)CHCCH is confirmed with IR spectroscopy. In separate studies of the (1 + 1) resonance-enhanced multiphoton ionization (REMPI) spectra, we observe the presence of C(6)H(5)OH in the molecular beam: C(6)H(5)OH + λ(275.1nm) → [C(6)H(5)OH Ã] + λ(275.1nm) → C(6)H(5)OH(+). From the REMPI and PIMS signals and previous work on methoxybenzene, we suggest that phenol results from a radical/radical reaction: CH(3) + C(5)H(4)OH → [CH(3)-C(5)H(4)OH]* → C(6)H(5)OH + 2H.

Comparison of the resonance-enhanced multiphoton ionization spectra of pyrrole and 2,5-dimethylpyrrole: Building toward an understanding of the electronic structure and photochemistry of porphyrins

The photophysical properties of porphyrins have relevance for their use as light-activated drugs in cancer treatment and sensitizers in solid-state solar cells. However, the appearance of their UV-visible spectra is usually explained inadequately by qualitative molecular-orbital theories. We intend to gain a better insight into the intense absorption bands, and excited-state dynamics, that make porphyrins appropriate for both of these applications by gradually building toward an understanding of the macrocyclic structure, starting with studies of smaller pyrrolic subunits. We have recorded the (1+1) and (2+1) resonance-enhanced multiphoton ionization (REMPI) spectra of pyrrole and 2,5-dimethylpyrrole between 25 600 cm(-1) (390 nm) and 48 500 cm(-1) (206 nm). We did not observe a (1+1) REMPI signal through the optically bright (1)B(2) (pipi( *)) and (1)A(1) (pipi( *)) states in pyrrole due to ultrafast deactivation via conical intersections with the dissociative (1)A(2) (pisigma( *)) and (1)B(1) (pisigma( *)) states. However, we did observe (2+1) REMPI through Rydberg states with a dominant feature at 27 432 cm(-1) (two-photon energy, 54 864 cm(-1)) assigned to a 3d<--pi transition. In contrast, 2,5-dimethylpyrrole has a broad and structured (1+1) REMPI spectrum between 36 000 and 42 500 cm(-1) as a result of vibronic transitions to the (1)B(2) (pipi( *)) state, and it does not show the 3d<--pi Rydberg transition via (2+1) REMPI. We have complemented the experimental studies by a theoretical treatment of the excited states of both molecules using time-dependent density functional theory (TD-DFT) and accounted for the contrasting features in the spectra. TD-DFT modeled the photochemical activity of both the optically dark (1)pisigma( *) states (dissociative) and optically bright (1)pipi( *) states well, predicting the barrierless deactivation of the (1)B(2) (pipi( *)) state of pyrrole and the bound minimum of the (1)B(2) (pipi( *)) state in 2,5-dimethylpyrrole. However, the quantitative agreement between vibronic transition energies and the excited-state frequencies calculated by TD-DFT was hampered by inaccurate modeling of Rydberg orbital mixing with the valence states, caused by the lack of an asymptotic correction to the exchange-correlation functionals used.

Resonance enhanced multi-photon ionization of neutral atoms sputtered with Ga-FIB

Resonance enhanced multi-photon ionization (REMPI) of neutral aluminum atoms sputtered with gallium focused ion beam (Ga-FIB) was studied in terms of substrate temperature and chemical state of the surfaces. Aluminum has the lowest excitation state (3p 2P 3/2) at 112 cm −1 above the ground state (3p 2P 1/2). The results showed that the total REMPI signal intensity of neutral aluminum atoms and the ratio of REMPI signal intensities attributed to 2P 1/2 to 2P 3/2 were increased at higher temperature. On the other hand, the REMPI signal and the ratio were decreased in the case of partially oxidized aluminum surfaces. Considering the result on Al 2O 3, it was confirmed that the REMPI signals of ground state 2P 1/2 and the first excited spate 2P 1/2 could be affected with surface oxidation state.

Formation of small PAHs in laminar premixed low-pressure propene and cyclopentene flames: Experiment and modeling

The formation of small PAHs with up to three rings has been studied in fuel-rich, nonsooting premixed flames of propene–oxygen–argon and cyclopentene–oxygen–argon at C/O = 0.77 and 50 mbar. Detailed species composition and temperature profiles for these flames had been studied before, using laser-induced fluorescence (LIF) and in situ molecular beam mass spectrometry (MBMS) with electron impact ionization (EI). These previous studies found a significant influence of the fuel structure upon the reaction pathways to benzene as the first aromatic ring. Here, the further molecular reaction sequence is studied with enhanced sensitivity, using MBMS coupled with resonance-enhanced multiphoton ionization (REMPI). Absolute concentrations of benzene were determined using both ionization strategies in order to establish a link to the previous investigations. The ratio of REMPI signals in the two flames for masses from m / z = 78 (benzene) up to approximately m / z = 200 is given and shows a predominant tendency of the cyclopentene flame to form aromatic species. The experimental results are complemented with simulations using two current chemical kinetic reaction models. The model predictions for important species in this mass range are discussed in the light of reaction flux and sensitivity analyses.

Experimental detection of rotational non-Boltzmann distribution in supersonic free molecular nitrogen flows

In the present study, we measure the rotational population in supersonic nitrogen-free jets using a resonantly enhanced multiphoton ionization (REMPI) method [Chem. Phys. Lett. 115, 492 (1985)], which is not influenced by secondary electrons, unlike the electron beam method. Nitrogen ions are detected as a REMPI signal and its spectra, depending on the wavelength of an irradiated laser beam, are analyzed to measure rotational temperature through the Boltzmann plot. Nitrogen gas expands into a vacuum chamber from a sonic nozzle with a D=0.50mm diameter, setting P0∙D (where P0 is the source pressure), depending inversely on the nozzle Knudsen number, at 15Torrmm or lower. For P0∙D=15Torrmm, the rotational temperature distribution along the centerline of the jet, measured by using only the linear portion of the Boltzmann plot lying at smaller rotational quantum numbers, coincides with Marrone’s data [Phys. Fluids 10, 521 (1967)] measured by electron beam fluorescence, and the rotational relaxation rate ZR calculated by the relaxation equation results in 1.3. However, the non-Boltzmann rotational distribution appears evidently in supersonic free molecular nitrogen flows for P0∙D⩽15Torrmm, in good agreement with the simulation results of the combination of the classical trajectory calculation and the direct simulation Monte Carlo method by Koura [Phys. Fluids 14, 1689 (2002)]. As a decrease in P0∙D, the deviation from the Boltzmann distribution and the partial freezing of the population arise more upstream.

Experimental Analysis of Non-Boltzmann Rotational Energy Distribution in Supersonic Free Molecular Flows

In the present study, we measure the rotational population in supersonic nitrogen free jets using a REMPI (resonantly enhanced multi-photon ionization) method, which is not influenced by secondary electrons, unlike an electron beam method. Nitrogen ions are detected as a REMPI signal and its spectra depending on the wavelength of an irradiated laser beam are analyzed to measure rotational temperature through the Boltzmann plot. Nitrogen gas expands into a vacuum chamber from a sonic nozzle with a D=0.50mm diameter, setting P0D (P0 : source pressure), depending irlversely on the nozzle Knudsen number, at 15 Torr·mm or lower. For PoD=15 Torr·mm, the rotational temperature distribution along the centerline of the jet, measured by Boltzmann plot, coincides with Marrone's data measured by electron beam fluorescence, and the rotational relaxation rate ZR calculated by the relaxation, equation results is 1.3. However, the non-Boltzmann rotational distribution appears evidently in the supersonic free molecular nitrogen flows for P0D≤15 Torr·mm and as a decrease in the P0D, the deviation from the Boltzmann distribution and the partial freezing of the population arise more upstream.

One- and two-photon excitation vibronic spectra of 2-methylallyl radical at 4.6–5.6 eV

Vibronically excited 2-methylallyl radical [CH2C(CH3)CH2] at 4.6–5.6 eV has been studied by 1+1 and 2+2 resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The 2-methylallyl radicals were produced by the flash pyrolysis of 3-bromo-2-methylpropene in a supersonic-jet expansion. The 2+2 REMPI spectrum of 2-methylallyl radical at 38 000–40 700 cm−1 is identified as B̃(1 2A1)←X̃(1 2A2) transition, i.e., the excitation of a nonbonding electron to the 3s Rydberg state (3s←n). Seven lowest-lying electronic states with excitation energy below 6 eV have been calculated in an MRCI level. Two new electronic bands have been observed at 38 500–41 000 cm−1 by 1+1 REMPI spectroscopy and assigned to C̃(1 2B2)←X̃(1 2A2) and Ẽ(2 2A2)←X̃(1 2A2). Much broader 1+1 REMPI signals at 41 000–43 5 00 cm−1 with HWHM of ∼80 cm−1 for each vibronic band could be due to D̃(2 2B2)←X̃(1 2A2) and/or F̃(3 2B2)←X̃(1 2A2) via an intensity borrowing from C̃(1 2B2)←X̃(1 2A2). Taking the computed geometries and vibrations of the ground- and excited electronic states, Franck–Condon factors (FCFs) have been calculated. Combining the FCFs with calculated excitation energies and oscillator strengths of the six electronic states at 4–6 eV, predicted spectral patterns have been used to assist spectroscopic analysis for the observed vibronic spectra of 2-methylallyl radical.

Near UV photodissociation of CD 3SCD 3: CD 3 fragment ( [formula omitted], [formula omitted]) vector correlations

The alignment of CD 3 fragments following the photodissociation of fully deuterated dimethyl sulfide (CD 3SCD 3) on the first absorption band (at 219 nm) has been characterised using (2 + 1) resonance-enhanced multiphoton ionization (REMPI) spectroscopy. Operation of the time-of-flight (TOF) mass spectrometer with a low ion extraction field rejects off-axis travelling CD 3 fragments, allowing velocity-selected REMPI spectra to be obtained. The strong dependence of the REMPI signal on the polarization of the probe laser is adequately explained by an impulsive model dissociation where the fragments recoil axially along the CD 3S–CD 3 bond. In general, the methyl fragments are found to be preferentially spinning about their C 3 axis after photodissociation, correlating with zero-point torsional vibrational motion of the parent molecule.

A Study on REMPI as a Measurement Technique for Highly Rarefied Gas Flows. Analyses of Experimental REMPI Spectra in Supersonic Free Molecular Flows.

A technique for measurement of thermodynamic variables with high sensitivity is necessary for analyses of highly rarefied gas flows, gas-surface interaction, and so on. REMPI (Resonantly Enhanced Multiphoton Ionization) is a powerful optical tool because of its high sensitivity even in highly rarefied gas flows and its ability to measure nonequilibrium amongst internal (translational, vibrational, and rotational) energy modes. In this paper, a REMPI system is developed and REMPI spectra are measured on the center line of a free-molecular flow. The fundamental properties of the REMPI signal are also described. A method using a Boltzmann plot with the spectral lines of multiple branches is proposed, and the ratio of the electronic transition dipole moments in the two-photon Honl-London factors for the O and P branches is determined.

Resonance enhanced multiphoton ionization as a diagnostic tool in glow discharge plasmas

Resonance enhanced multiphoton ionization (REMPI) has been used to quantitatively detect benzene in an Ar dc plasma. The plasma reactor was fabricated from a 5 cm diam Pyrex tube that was about 30 cm long. The reactor also possessed two plasma electrodes that were separated by 3 cm. The plasma was generated at 160 mTorr pressure under continuous flow of Ar that was seeded with 100–1000 ppm of benzene. Ions generated by REMPI were detected using a separate electrode placed at the edge of the plasma zone. We were able to obtain well-resolved and unambiguous benzene REMPI spectra in the 250–265 nm laser wavelength under a broad range of plasma conditions. The spectra obtained were well resolved, exhibited sharp spectral features and were in complete agreement with the results reported in the literature under nonplasma conditions. Measured REMPI signal intensities also were linearly proportional to the benzene concentration, suggesting that REMPI can be a useful quantitative plasma diagnostic.

A Study on REMPI as a Measurement Technique for Highly Rarefied Gas Flows. Analyses of Experimental REMPI Spectra in Supersonic Free Molecular Flows.

A measuremnt technique of thermodynamic variables with high sensitivity is strongly demanded for analyses of highly rarefied gas flows, gas-surface interaction, and so on. REMPI (Resonantly Enhanced Multiphoton Ionization) is a powerful optical tool because of its high sensitivity even in highly rarefied gas flows and ability to measure nonequilibrium among internal (translational, vibrational, and rotational) energy. In this paper, the MEMPI system is constructed and the REMPI spectra are measured at the center line of a free molecular flow. The fundamental properties of REMPI signal are also described. A method of Boltzmann plot using the spectral lines of different branches is proposed, determining electronic transition dipole moments in Honl-London factors.

The UV photodissociation of HI revisited: REMPI measurements of [formula omitted] atom spin–orbit branching fractions

The UV photodissociation dynamics of HI has been studied at 5 photolysis wavelengths, λ=222, 248, 258, 266 and 280 nm, by measuring branching fractions for formation of spin–orbit excited I( 2 P 1/2) atoms using (2+1) resonance enhanced multi-photon ionization (REMPI) detection of I( 2 P 3/2) and I( 2 P 1/2) . Branching fractions are derived from experimental REMPI signals using an independently established calibration factor, and the results are compared with previous determinations based on different H-atom detection schemes. Discrepancies between these previous measurements for photolysis wavelengths longer than 220 nm are resolved by comparison with the I-atom REMPI results and by detailed modelling of the H-atom detection experiments previously performed in this laboratory. The branching fraction for I( 2 P 1/2) peaks at a value of 0.50±0.05 for wavelengths around 250 nm.

Spin–orbit branching in Cl(2P) atoms produced by ultraviolet photodissociation of HCl

The ultraviolet photodissociation of jet-cooled HCl molecules at 5 wavelengths in the range 201–210 nm has been investigated. Ground state hydrogen photofragments, H(2S), were detected using the H Rydberg atom time-of-flight (HRTOF) technique to obtain directly the relative yields of the available product channels: H(2S)+Cl(2P3/2) and H(2S)+Cl(2P1/2). The product branching fractions are reported and compared with recent experimental measurements and theoretical calculations. In addition, the two spin–orbit components of ground state chlorine photofragments formed by photolysis of HCl at 205.5 nm were monitored using (2+1) resonance enhanced multiphoton ionization (REMPI). The relative sensitivity of this detection method for Cl(2P3/2) and Cl(2P1/2) atoms is found by comparing the relative REMPI signal intensities with the product branching fraction determined by the HRTOF technique.

Detection of DCl by multiphoton ionization and determination of DCl and HCl internal state distributions

A study of the 2+1 resonantly enhanced multiphoton ionization (REMPI) spectrum of DCl is reported. Transition energies for excitation of the F 1Δ–X 1Σ+ (0,0) and (1,0) bands, as well as the V 1Σ+–X 1Σ+(v′,0) bands, for v′=15–19, are presented. The derived molecular constants for the F–X (0,0) and the V–X bands agree well with those previously obtained from analysis of the one-photon VUV absorption spectrum [A. E. Douglas and F. R. Greening, Can. J. Phys. 57, 1650 (1979)]. The ion signals for excitation through various rotational lines in the E–X (0,0) and F–X (0,0) and (1,0) bands are compared with theoretical two-photon line strengths. Extensive power- and J′-dependent ion fragmentation is observed for the former band. No fragmentation is observed in the F–X bands; however, the ion signal strengths are found to vary strongly with J′. This variation of REMPI signal strengths vs J′ was shown to be due to an indirect predissociation, as in HCl. Tables of experimental line strength factors for the F–X (0,0) and (1,0) bands of HCl and DCl are reported. Finally, the relative REMPI detection sensitivities for HCl and DCl, through their respective F–X (0,0) R(1) lines, are compared.

Determination of the Rotational and Vibrational Constants of Photolytically Generated CF Radicals by REMPI Measurements

The detailed spectroscopic study of transient free CF radicals with the resonance enhanced multiphoton ionization (REMPI) method is the aim of this paper. The CF radicals were generated from the gaseous (50—200 Pa) halocarbons CCl3F, CCl2F2, CClF3, 1,2-C2Cl2F4, C2ClF3, and C6F6by a combined infrared and UV/VIS laser photolysis. Because of the small absorption of the halocarbon molecules in the dye laser range used (328—485 nm) a preceding production of precursors (capable of absorbing dye laser photons) was necessary, which was achieved by IR multiphoton dissociation of the parent molecules with a TEA CO2laser pulse. In a subsequent absorption of UV/VIS dye laser photons the precursors were photolyzed yielding CF radicals. These could be spectroscopically investigated by their REMPI signals in a (2 + 1)-photon absorption process in the focussed radiation of the same dye laser pulse. About 20 vibrational bands were measured, rotationally resolved, and were assigned to a two-photon 3pπD2Πr←←X2Πrresonant transition followed by one-photon absorption for direct ionization of CF. Compared to the cold CF radicals, commonly extracted from a flow reactor, the laser-photolytically generated CF radicals were found to be vibrationally and rotationally extremely excited (v″ ≤ 12,J≤ 80.5). Using the wide rotational contour of the vibrational bands we could determine with greater precision than before the electronic, vibrational, and rotational constantsTe, ωe, ωexe, ωeye,Av,Bv, andDv, respectively, for the Rydberg state 3pπD2Πr(v′ = 0—6) and the electronic ground stateX2Πr(v″ = 0—12). Based on these constants we calculated the Morse and the Dunham potential curves for both states. A detailed study of missing and shifted rovibronic lines of theDstate yielded first information on the vibrational levelsv′ = 3, 4, and 5 of theB2Δrvalence state, which cannot be measured directly by REMPI. Additionally, we measured for the first time in the UV wavelength range a (1 + 1)-photon REMPI spectrum belonging to the vibrational bandsv′ = 0 ←v″ = 0 andv′ = 1 ←v″ = 0 of theA2Σ+←X2Πrtransition. Even in the range of high rotational quantum numbersJwe found no indication of Λ-type doubling of the rovibronic states within a resolution of 0.2 cm−1.