Observation of the 3(3)Sigma(+)-X1Sigma+ transition of KRb by resonance enhanced two-photon ionization in a pulsed molecular beam: Hyperfine structures of 39K85Rb and 39K87Rb isotopomers

The 3(3)Sigma(+)-X1Sigma+ transition of KRb is observed by resonance enhanced two-photon ionization in a pulsed molecular beam. Hyperfine splittings of 39K85Rb and 39K87Rb isotopomers are observed. From the magnitude of hyperfine splittings, we found that the main hyperfine structure was dominated by the Fermi contact interaction between the Rb nuclear spin and the unpaired electron spin. The Fermi contact interaction constants were determined to be 291 MHz for 39K85Rb and 665 MHz for 39K87Rb. In the KRb 3(3)Sigma+ state the electron spin couples more strongly with the Rb nuclear spin than with other angular momenta, and the energy level structure is well described by the hyperfine angular momentum coupling scheme of the b(betaS) case. The molecular constants and the Rydberg-Klein-Rees potential energy curve of the 3(3)Sigma+ state were determined.     

Multiphoton ionization: a method for characterizing molecular beams and beam reaction products

The technique of multiphoton ionization is applied to Na₂ in an effusive and a supersonic beam to characterize the internal state distribution of this molecule. A tunable dye laser is swept through the wavelength region of the Na₂ BX system and positive ions are counted as a function of laser wavelength. Multiphoton ionization involving a real intermediate state is also used to characterize the BaCl product formed in the beam-gas reaction Ba + HCl. This detection system can combine mass spectrometry with laser spectroscopy for the identification and characterization of beam species. Advantages and drawbacks are discussed.     

Mass-resolved resonance enhanced ionization study of complicated excited electronic states of Rb2 near 430 nm and their predissociation dynamics

We have investigated the Rb2 430 nm system by resonance enhanced two-photon ionization and photofragment yield spectroscopy. Four electronically excited states have been assigned, and their two-channel (fast and slow) predissociation has been observed. For the 3 1Piu, 3 1Sigmau+, 3 3Piu(0u+), and 3 3Piu(1u) states, electronic term values (Te) and vibrational constants (omegae and omegaexe) have been determined. For the predissociation dynamics, we have observed the fast predissociation threshold between the isotopically shifted 3 1Piu v' = 14 levels of 85Rb2 and 85Rb87Rb where the 4 2D5/2 + 5 2S1/2 atomic fine-structure limit is located. The 1 3Deltau state corresponding to the 4 2D5/2 + 5 2S1/2 atomic fine-structure limit has been assigned to the predissociating perturber responsible for the fast channel. Also, we have found that the fast channel branches out into two finer product channels due to long-range potential crossing.     

Excitation, ionization, and fragmentation of chiral molecules in asymmetric microenvironments: a mass-resolved R2PI spectroscopic study

One- and two-color, mass selected R2PI spectra of the S(1) <-- S(0) transitions in the bare (R)-(+)-1-phenyl-1-propanol and its complexes with bidentate solvent molecules, like the (R)-(-)- and (S)-(+)-3-hydroxytetrahydrofuran enantiomers, have been recorded after a supersonic molecular beam expansion. The one-color R2PI excitation spectra of the diastereomeric complexes are characterized by three main peaks, one red-shifted and the other two blue-shifted relative to the band origin of the most stable anti conformer of the bare chromophore. The opposite direction of these spectral shifts is ascribed to the occurrence of three different hydrogen bonded isomeric structures for each individual complex, while their different magnitude depends on the configuration of the bidentate solvent molecule as well as its specific hydrogen bond interaction center, whether the ethereal oxygen atom or the hydroxyl group. The same factors play a major role in determining the magnitude of the phenomenological activation barriers for the loss of an ethyl radical from the ionized diastereomeric complexes.     

Photodissociation of Cl2SO at 248 and 193 nm in a molecular beam

A pulse molecular beam of Cl₂SO was photodissociated at 248 and 193 nm. The time-of-flight distributions were observed for the photofragments, Cl, ClSO and SO. The primary processes are Cl + ClSO (I), 2Cl + SO (II) and Cl₂ + SO (III). At 193 nm the measured translational energy distributions imply a vibrationally excited ClSO fragment in process (I), and a simultaneous dissociation in process (II). The relative quantum yield is φ_I < φ_II. At 248 nm a radical process (I) is dominant compared to a molecular process (III).     

Interpreting closed-loop learning control of molecular fragmentation in terms of wave-packet dynamics and enhanced molecular ionization

We interpret a molecular fragmentation experiment using shaped, ultrafast laser pulses in terms of enhanced molecular ionization during dissociation. A closed-loop learning control experiment was performed to maximize the CF3+CH3+ production ratio in the dissociative ionization of CH3COCF3. Using ab inito molecular structure calculations and quasistatic molecular ionization calculations along with data from pump-probe experiments, we identify the primary control mechanism which is quite general and should be applicable to a broad class of molecules.     

Production of metal atoms,clusters and complexes in pulsed discharge-excited jets : Studies via mass-resolved laser multiphoton ionization

A pulsed, high-voltage, discharge-excited nozzle source has been developed and exploited to study the possibility of sputtering and entraining various metal atoms into the gas expansion from the discharge electrodes. By appropriate choice of electrode materials, atomic beams of copper, silver, tin and lead have been generated and spectroscopically characterized by 2 + 1 laser multiphoton ionization with mass analysis. Sufficiently high atomic densities are achieved with this nozzle system that metal clustering also takes place, producing dimers such as Ag₂, species of mixed composition such as AgSn and trimers, Ag₃. In addition, chemical reactions have been observed which can be initiated in the discharge and which lead to the formation of metal–ligand complexes when suitable molecules are seeded into the carrier gas. Mass spectral evidence for two such silver complexes is presented.     

Determination of the first ionization energy of furan (C4H4O) from an extrapolation of two nd Rydberg series observed in the mass-resolved (2 + 1) resonance enhanced multiphoton ionization spectrum

The first ionization energy of furan (C₄H₄O) has been determined from a short extrapolation of two nd (n=6–22) Rydberg series observed in the mass-resolved (2 + 1) resonance enhanced multiphoton ionization spectrum as IE=71673 ± 3 cm⁻¹. This value confirms the higher of the two values in the literature.     

Two-color resonance enhanced multiphoton ionization of Van der Waals molecules: studies of spectroscopy shifts and ionization thresholds of paraxylene clustered with argon

Van der Waals molecules comprised of paraxylene (PX) clustered with Ar atoms were formed in pulsed supersonic jet, subjected to two-color resonance enhanced two-photon ionization (2C R2PI), and analyzed in a time-of-flight mass spectrometer. Perturbed S₁ states of the complexes display spectral shifts relative to the nascent molecule (PX). The appearance potentials (AP) of PX·^Ar_n clusters were measured for n = 1 to 6. On average, the AP is red-shifted ≈ 120 cm⁻¹ per Ar atom addition, but there are deviations from a smooth trend at the dimer and pentamer.     

A molecular beam study of a van der Waals complex; vibration-inversion transitions in NH3-Ar

Three ν₂ vibration-inversion transitions of NH₃-Ar have been found with linewidths smaller than 2 MHz, yielding a minimum predissociation lifetime of 80 ns. While one of these lines was previously identified as a low frequency inversion doublet transition, the other two lines belong to the high frequency component of the ν₂ vibration-inversion doublet.     

Signal transduction in a coupled hormone system: selective explicit internal signal stochastic resonance and its control

Cooperative interactions of signal transduction and environmental noise are investigated with a coupled hormone system, in which selective explicit internal signal stochastic resonance (EISSR) is observed. More specifically, the large peak of a period-2 oscillation (i.e., a strong signal) is greatly amplified by the environmental noise while the small peak (i.e., a weak signal) does not exhibit cooperative interactions with noise. The EISSR phenomenon could be controlled by adjusting the frequency or amplitude of an external signal and a critical amplitude for external signal is found. Significantly, the maximal signal-to-noise ratio increases almost linearly with the increment of control parameter, despite that the magnitude of the large peak is decreased. In addition, the noise does not alter the fundamental frequencies of the strong signal and the weak signal, which implicates that the system can keep its intrinsic oscillatory state and resist the effect of environmental fluctuations.     

Gel-surface enhanced fluorescence sensing system coupled to a continuous-flow assembly for simultaneous monitoring of benomyl and carbendazim

A novel, versatile and sensitive continuous-flow on-line solid phase fluorescence based system is proposed for the simultaneous determination of benomyl and carbendazim. The continuous-flow system is based on the on-line preconcentration and resolution of the pesticides on a solid sensing zone, followed by the sequential measure of their native fluorescence, monitored at 235/306 and 293/398 nm (λ_exc/λ_em for carbendazim and benomyl, respectively), and later desorption of these analytes (from the flow-through cell filled with C₁₈ silica gel) using aqueous methanol mixtures as carrier and eluent solutions.A double discrimination is used for the simultaneous monitoring of these analytes: (1) the usage of two pair of excitation/emision wavelengths, performed by the use of a multiwavelength fluorescence detection mode and (2) a temporary sequentiation in the arrival of the analytes to the sensing system by on-line separation due to the different kinetics showed by the analytes in the sorption-desorption process performed just in the solid support placed in the flow-through cell. Carbendazim is determined the first, because it shows a weaker retention in the C₁₈ bonded phase silica beads, while benomyl is strongly fixed. Then, benomyl is conveniently eluted from the flow-through sensing zone and its native fluorescence signal is measured (at 398 nm). The sensor was calibrated for two different injection volumes: 400 and 2000 μl. Using a 2000 μl sample volume, the analytical signal showed linearity in the range 0.050-1.0 and 0.020-0.50 μg ml⁻¹ with detection limits of 3.0 and 7.5 ng ml⁻¹ for carbendazim and benomyl, respectively, and R.S.D. values smaller than 2% for both analytes. A recovery study was performed on four different spiked environmental water samples at concentration levels from 0.05 to 0.35 μg ml⁻¹. The recovery percentage ranged from 97 to 104%, and from 98 to 104%, for benomyl and carbendazim, respectively.     

Magnetic properties of iron clusters in a molecular beam: resolution of a controversy

Magnetic properties of small iron clusters in a supersonic molecular beam are investigated. The magnetization is probed as function of magnetic field, temperature and cluster size. Temperatures are controlled by changing the source temperature (100 K to 500 K) and the expansion conditions. The clusters also may be heated in flight with light from a pulsed laser. Hot clusters are found to be superparamagnetic however cold clusters are not but show strongly reduced magnetization which furthermore is non-linear with the applied field. Experimentally it is found that the anomalies are related to the cluster rotations. We also address a controversy between our earlier findings [1] and those by Bucher et al. [2], and demonstrate that their temperature determinations and consequent conclusions are incorrect.     

In vitro evolution of molecular cooperation in CATCH, a cooperatively coupled amplification system

Background: One of the key issues in the investigation of evolution is how complex systems evolved from simple chemical replicators. Theoretical work proposed several models in which complex replicating systems are kinetically stabilized. The development of powerful isothermal amplification technique allows complex nucleic acid based evolving in vitro systems to be set up, which may then serve to verify experimentally current theories of evolution. Recently such a system based on the 3SR (self-sustained sequence replication) reaction has been established to investigate the evolution of cooperation: the trans-cooperatively coupled CATCH (cooperative amplification by cross hybridization).Results: Over four rounds of serial transfer, the cooperatively coupled two species CATCH system evolved into a more complex cooperative four species system, which then was overgrown by CATCH-derived RNA-Z-like hairpin species. In contrast to the classical RNA-Z species, these molecules have complementary loop sequences and self-amplify using a dual mechanism that includes concentration-dependent phases of noncooperative and cooperative amplification.Conclusions: The evolution of a cooperative system, under conditions that were alternately unfavorable and favorable for cooperative amplification, led to a system showing facultative cooperation. This principle of facultative cooperation preserves the complexity of the system investigated and could have general implications for the evolution and stabilization of cooperation under oscillating reaction conditions.     

Mobile resonance enhanced multiphoton ionisation-time-of-flight mass spectrometer with a novel hybrid laser desorption/molecular beam ion source for rapid detection of aromatic trace compounds from gas phase and solid samples

A mobile, field applicable resonance enhanced multiphoton ionisation-time-of-flight mass spectrometer (REMPI-TOFMS) for direct-inlet gas-phase analysis as well as for characterisation of solid samples, has been developed. For solid-sample analysis, laser desorption (LD) with IR-laser pulses is used for volatilisation of the neutral analytes from a target in the ion source prior to REMPI-TOFMS analysis. For direct inlet gas-phase analysis, a needle gas inlet is used and the formed molecular beam is REMPI-ionised directly in the ion source. A special ion source was developed, which can be rapidly interchanged between the REMPI-TOFMS and the LD-REMPI-TOFMS modes. The system is equipped with a tuneable optical parametrical oscillator (OPO)-laser system. The analytical properties are characterised for both modes of operations. In both cases, analytical performance is similar to that achieved with instruments specifically designed for either gas-phase or solid-phase characterisation. Results from field measurements of a wood combustion flue gas of a 174 kW bio mass furnace are given. Gas-phase analysis was applied for time-resolved on-line REMPI-TOFMS of the semi-volatile aromatic flue gas constituents. The low-volatile aromatic compounds, which were precipitated with the particulate matter in the filtering unit of the sampling train, were characterised by direct LD-REMPI-TOFMS of the glass fibre filter material. By combining the gas-phase and particulate results a rather comprehensive characterisation of the aromatic species present in the flue gas (i.e. aerosol--gas phase plus particulate matter) was achieved.     

Dynamics of Cl (2Pj) atom formation in the photodissociation of fumaryl chloride (ClCO - CH = CH - COCl) at 235 nm: a resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) study

The photodissociation dynamics of fumaryl chloride (ClCO-CH═CH-COCl) has been studied in a supersonic molecular beam around 235 nm using resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) technique by detecting the nascent state of the primary chlorine atom. A single laser has been used for excitation of fumaryl chloride and the REMPI detection of chlorine atoms in their spin-orbit states, Cl ((2)P(3/2)) and Cl* ((2)P(1/2)). We have determined the translational energy distribution, the recoil anisotropy parameter, β, and the spin-orbit branching ratio for chlorine atom elimination channels. To obtain these, measured polarization-dependent and state-specific TOF profiles are converted into kinetic energy distributions, using a least-squares fitting method, taking into account the fragment recoil anisotropies, β(i). The TOF profiles for both Cl and Cl* are found to be independent of laser polarization; i.e., β is well characterized by a value of 0.0, within the experimental uncertainties. Two components, namely, the fast and the slow, are observed in the translational energy distribution, P(E(T)), of Cl and Cl* atoms, and assigned to be formed from different potential energy surfaces. The average translational energies for the fast components of the Cl and Cl* channels are 14.9 ± 1.6 and 16.8 ± 1.6 kcal/mol, respectively. Similarly, for the slow components, the average translational energies of the Cl and Cl* channels are 3.4 ± 0.8 and 3.1 ± 0.8 kcal/mol, respectively. The energy partitioning into the translational modes is interpreted with the help of various models, such as impulsive and statistical models. Apart from the chlorine atom elimination channel, molecular hydrogen chloride (HCl) elimination is also observed in the photodissociation process. The HCl product has been detected, using a REMPI scheme in the region of 236-237 nm. The observation of the molecular HCl in the dissociation process highlights the importance of the relaxation process, in which the initially excited parent molecule relaxes to the ground state from where the molecular (HCl) elimination takes place.     

Photodissociation and photoisomerization of alpha-fluorotoluene and 4-fluorotoluene in a molecular beam

The photodissociation of jet-cooled alpha-fluorotoluene and 4-fluorotoluene at 193 and 248 nm was studied using vacuum ultraviolet (vuv) photoionization/multimass ion imaging techniques as well as electron impact ionization/photofragment translational spectroscopy. Four dissociation channels were observed for alpha-fluorotoluene at both 193 and 248 nm, including two major channels C6H5CH2F-->C6H5CH2 (or C7H7)+F and C6H5CH2F-->C6H5CH (or C7H6)+HF and two minor channels C6H5CH2F-->C6H5CHF+H and C6H5CH2F-->C6H5+CH2F. The vuv wavelength dependence of the C7H7 fragment photoionization spectra indicates that at least part of the F atom elimination channel results from the isomerization of alpha-fluorotoluene to a seven-membered ring prior to dissociation. Dissociation channels of 4-fluorotoluene at 193 nm include two major channels C6H4FCH3-->C6H4FCH2+H and C6H4FCH3-->C6H4F+CH3 and two minor channels C6H4FCH3-->C6H5CH2 (or C7H7)+F and C6H4FCH3-->C6H5CH (or C7H6)+HF. The dissociation rates for alpha-fluorotoluene at 193 and 248 nm are 3.3 x 10(7) and 5.6 x 10(5) s(-1), respectively. The dissociation rate for 4-fluorotoluene at 193 nm is 1.0 x 10(6) s(-1). An ab initio calculation demonstrates that the barrier height for isomerization from alpha-fluorotoluene to a seven-membered ring isomer is much lower than that from 4-fluorotoluene to a seven-membered ring isomer. The experimental observed differences of dissociation rates and relative branching ratios between alpha-fluorotoluene and 4-fluorotoluene may be explained by the differences in the six-membered ring to seven-membered ring isomerization barrier heights, F atom elimination threshold, and HF elimination threshold between alpha-fluorotoluene and 4-fluorotoluene.