Free Jet Expansion Research Articles

Potentials of the D10u+ (61S0) and F31u(63P2) Electronic Rydberg States of Cd2 from ab Initio Calculations and Laser-Induced Fluorescence Excitation Spectra

The method of supersonic free-jet expansion beam combined with techniques of laser spectroscopy was used in an investigation of vibronic and isotopic structures in the D¹0(u)⁺ (6¹S₀) and F³1(u)(6³P₂) electronic energy Rydberg states of Cd₂. Laser-induced fluorescence excitation spectra recorded using the D¹0(u)⁺ ← X¹0(g)⁺(5¹S₀) and F³1(u) ← X¹0(g)⁺ transitions in the region of 206-218 nm provided spectroscopic characteristics of the excited states and allowed constructing of their intratomic potentials. Isotopic structures recorded in the (υ',υ'') bands of the D¹0(u)⁺ ← X¹0(g)⁺ transition were used in determination of the D¹0(u)⁺ state vibrational characteristics (ω'(e)x'(e), ω'(e)x'(e)) and υ' assignment. The ν(0,0) recorded directly in the F³1(u) ← X¹0(g)⁺ transition enabled determination of the bottom of the F³1(u) state potential well. Valence ab initio calculations of Cd₂ interatomic potentials were performed with relativistic and spin-orbit effects taken into account. The experimental results were compared with results of the ab initio calculations. A free-jet expansion of Cd₂ as a source of entangled atoms for a test of Bell's inequality was analyzed.

Laser ablation with resonance-enhanced multiphoton ionization time-of-flight mass spectrometry for determining aromatic lignin volatilization products from biomass

We have designed and developed a laser ablation∕pulsed sample introduction∕mass spectrometry platform that integrates pyrolysis (py) and∕or laser ablation (LA) with resonance-enhanced multiphoton ionization (REMPI) reflectron time-of-flight mass spectrometry (TOFMS). Using this apparatus, we measured lignin volatilization products of untreated biomass materials. Biomass vapors are produced by either a custom-built hot stage pyrolysis reactor or laser ablation using the third harmonic of an Nd:YAG laser (355 nm). The resulting vapors are entrained in a free jet expansion of He, then skimmed and introduced into an ionization region. One color resonance-enhanced multiphoton ionization (1+1 REMPI) is used, resulting in highly selective detection of lignin subunits from complex vapors of biomass materials. The spectra obtained by py-REMPI-TOFMS and LA-REMPI-TOFMS display high selectivity and decreased fragmentation compared to spectra recorded by an electron impact ionization molecular beam mass spectrometer (EI-MBMS). The laser ablation method demonstrates the ability to selectively isolate and volatilize specific tissues within the same plant material and then detect lignin-based products from the vapors with enhanced sensitivity. The identification of select products observed in the LA-REMPI-TOFMS experiment is confirmed by comparing their REMPI wavelength scans with that of known standards.

Structure and hydrogen bond vibrations of the jet-cooled 1:1 complex between 7-azaindole and formamide: A laser-induced fluorescence spectroscopy study

Laser-induced fluorescence spectra of 7-azaindole ⋯ formamide complex are measured in a supersonic free jet expansion. Calculation at MP2/6-311++G ∗∗ level predicts a cyclic doubly hydrogen-bonded structure for the complex is favoured most in the ground state. The complex emits only UV fluorescence from the locally excited state and tautomerization is inhibited under the jet-cooled environment. This photophysical behavior is consistent with the predictions of CIS/6-311++G ∗∗ and TDDFT/6-311++G ∗∗ calculations. The low-frequency bands in the fluorescence spectra are assigned to different hydrogen bond modes of the complex, and the spectra reveal signatures of mixing between inter- and intra-molecular vibrational modes in the excited state.

“Union is strength”: how weak hydrogen bonds become stronger

Recently reported rotational spectroscopic studies on small dimers and oligomers bound by weak hydrogen bonds show that the driving forces, the spatial arrangement and the dynamical features displayed are very different from those involved in stronger and conventional hydrogen bonds. The very small binding energies (similar to those of van der Waals interactions) imply that the stabilization of the dimer is often obtained by networks of weak hydrogen bonds. Even in the presence of multiple bonds the partner molecules show a high degree of internal freedom within the complex. This paper analyses several examples of molecular adducts bound by weak hydrogen bonds formed in free jet expansions and recently characterized by rotational spectroscopy. They include weakly bound complexes of weak donors with strong acceptors (C-H···O,N, S-H···O,N), strong donors (O-H, N-H) with weak acceptors such as the halogen atoms and π systems but also the elusive interactions between weak donors and weak acceptors (C-H···π and C-H···halogen). Examples are also given where rotational spectroscopy highlights that weak hydrogen bonds are extremely important in chiral recognition phenomena and as driving forces of the conformational landscape of important biomolecules.

High resolution overtone spectroscopy of the acetylene van der Waals dimer, (12C2H2)2

CW-cavity ring down spectroscopy was used to record in a free jet expansion the spectrum of the absorption band in ((12)C(2)H(2))(2) with origin at 6547.6 cm(-1). It is a perpendicular band and corresponds to 2CH excitation in the hat unit of the T-shaped dimer. Calibration (better than ±1 × 10(-3) cm(-1) accuracy) and ring-down time (130 μs) were improved compared to a previous contribution (Didriche et al. Mol. Phys., 2010, 108, 2158-2164). A line-by-line analysis was achieved. Three series of lines were identified involving levels with A(1)(+), E(+) and B(1)(+) ground state tunneling symmetries, confirming the spectral and symmetry analyses reported in the literature for the 1CH excitation band (Fraser et al. J. Chem. Phys., 1988, 89, 6028-6045). 164 vibration-rotation-tunneling lines were assigned in the K(a)' - K(a)'' = 2 - 3, 0 - 1, 2 - 1 and 4 - 3 sub-bands and effective rigid rotor vibration-rotation constants were obtained by simultaneously fitting 1CH and 2CH lines with the same symmetry series. Perturbations affecting the K(a) stacks, in particular, are reported. The tunneling frequency in 2CH is estimated to be ν(tunn)(2CH) = 270 MHz for the K(a) = 0 stack. The rotational temperature is determined to be 23 K from relative line intensities and the lifetime of the dimer in the 2CH hat state is estimated to be 1 ns from individual line widths.

Structure of vibrational bands of the [formula omitted] (53P1), B3[formula omitted] (53P1) transitions in CdAr and CdKr studied by optical–optical double resonance method

Isotopic and rotational structures of the (υ′,υ″)=(13,5), (14,5), (16,5) and (υ′,υ″)=(0,1)–(6,1) vibrational bands of the E3Σ+←A3Π0+ and E3Σ+←B3Σ1+ transitions in CdAr, respectively, as well as the (υ′,υ″)=(21,9) of the E3Σ+←A3Π0+ transition in CdKr were investigated using free-jet expansion beam and laser excitation. An optical–optical double resonance process was employed from the X1Σ0++ to the E3Σ+ Rydberg via the A3Π0+ or B3Σ1+ intermediate electronic state. The structure of the bands with resolved isotopic structure was analyzed including their rotational structure. Analysis and simulation of the bands provided values for vibrational and rotational characteristics of the states. The analysis extended our previous low-resolution studies performed for the E3Σ+←A3Π0+ and E3Σ+←B3Σ1+ transitions.

Electronic transitions of iridium monoboride

Electronic transition spectrum of iridium monoboride in the spectral region between 400 and 545nm has been recorded and analyzed using laser vaporization/reaction free jet expansion source and laser induced fluorescence spectroscopy. Thirteen vibrational bands belonging to four electronic transitions, namely the [18.8]3Δ3 – X3Δ3, [21.1]3Φ4 – X3Δ3, [22.8]3Φ3 – X3Δ3, and [22.4]1Φ3 – a1Δ2 transitions, have been identified and rotationally analyzed. Spectra of all four isotopic molecules were observed.

Numerical simulation of supercritical carbon dioxide flows across critical point

Numerical study of supercritical carbon-dioxide flows across the critical point is presented. The present numerical method is based on the preconditioning method developed by Yamamoto and mathematical models of thermophysical properties for carbon dioxide programmed in the program package for thermophysical properties of fluids, developed by Kyushu University. First, the two-dimensional natural convection of carbon dioxide between two parallel plates is calculated while changing the bulk pressure. The calculated thermophysical properties of the carbon-dioxide flow under supercritical pressure are compared with those in a gas condition. Next, the natural convection of carbon dioxide in an O-shaped cyclic channel is calculated, and the effect of the density difference induced by the phase change to the flow is investigated. For application to high-speed flows, supercritical carbon dioxide flows through a nozzle with free-jet expansion (known as the process of rapid expansion of supercritical solutions) process are calculated. The calculated shock distance to the Mach disk generated in the free jet is compared with experiments and the density variations in the nozzle while changing the inlet temperature are numerically predicted.

Note: A modular and robust continuous supersonic expansion discharge source

A direct current discharge has been coupled with a continuous supersonic expansion to provide a source of rotationally cold molecular ions for gas phase spectroscopy. Constructed primarily of machinable ceramic and stainless steel, this source design is modular, customizable, and robust. Its performance has been assessed by recording transitions within the nu(2) fundamental band of H(3) (+) using cavity ringdown spectroscopy to determine the rotational temperature of ions produced in the free-jet expansion. Temperature and column density were recorded as a function of discharge current as the source was operated over a period of 200 h. Observed temperatures ranged between 50-110 K, and the ion column densities between 8x10(10) and 2x10(12) cm(-2).

Laser induced fluorescence spectroscopy of iridium monophosphide

Electronic transition spectrum of IrP in the spectral region between 390 and 471 nm has been recorded and analyzed using laser vaporization/reaction free jet expansion and laser induced fluorescence spectroscopy. Reacting laser-ablated iridium atoms with 1% PH 3 seeded in argon produced the IrP molecule. Five electronic transition systems were identified and rotationally resolved transition lines of both the 191IrP and 193IrP isotopes were analyzed. Least squares fit of the rotational lines yielded molecular constants for the upper and lower states.

An experimental investigation on the performance of conical nozzles for argon cluster formation in supersonic jets

This work intends to get a better understanding of cluster formation in supersonic nozzles of different geometries. The throat diameters d are within 0.26 mm < or = d < or = 0.62 mm, the half-opening-angle alpha within 4.2 degrees < or = alpha < or = 11.3 degrees, and the length L of the conical section is 17.5 mm (eight nozzles) or 12 mm (two nozzles). Thus the so-called "equivalent sonic-nozzle diameter d(eq)" for these conical nozzle geometries, defined by d(eq)=0.74 d/tan alpha (for monatomic gases), is in the range of 1.59 mm < or = d(eq) < or = 5.21 mm. Source temperature for the clustering experiments was T(0)=298 K, and the backing pressure P(0) was between 0.5 and 30 bars. The (average) cluster sizes observed for these conical nozzles deviate from the predictions of the simple stream-tube-model. These deviations are accounted for by introducing the so-called "effective equivalent sonic-nozzle diameter d(eq)*," defined as the product of the equivalent sonic-nozzle diameter d(eq) and a new parameter delta, d(eq)*=deltad(eq). The parameter delta serves to modify the equivalent diameters d(eq) of the conical nozzles, which are applied in the idealized cases where the gas flows are suggested to be formed through free jet expansion. Then, delta represents the deviation of the performance in cluster formation of the practical conical nozzles from those predicted based on the idealized picture. The experimental results show that the values of delta can be described by an empirical formula, depending on the gas backing pressure P(0) and the parameter d(eq) of the conical nozzles. The degradation of the performance of the present conical nozzles was found with the increase in P(0) and the larger d(eq). It was revealed that delta is inversely proportional to a fractional power (approximately 0.5-0.6) of the molecular density n(mol) in the gas flows under the present experimental conditions. The boundary layers effects are considered to be mainly responsible for the restriction of the performance of the conical nozzles in cluster formation.

Cavity Ring Down Laser Absorption Spectroscopy of NiI

The absorption spectrum of NiI between 445 and 510 nm has been investigated using the technique of laser vaporization/reaction with free jet expansion and cavity ring down laser absorption spectroscopy. Two new transitions namely, [21.3]25/2-X25/2 and [21.9]23/2-X25/2 systems were identified and studied. Spectra of both 58NiI and 60NiI isotopic molecules were observed. Equilibrium molecular constants for both electronic states are reported and the equilibrium bond length for the [21.3]25/2 state and the [21.9]23/2 state was respectively determined to be 2.431 and 2.481 A.

Laser spectroscopy of NiI: New electronic states and hyperfine structure

Two new electronic transition systems, namely, the [14.0](2)Phi(7/2)-Chi (2)Delta(5/2) and the [15.7](2)Phi(5/2)-Chi (2)Delta(5/2) transitions were observed and analyzed using laser vaporization/reaction supersonic free jet expansion and high resolution laser induced fluorescence spectroscopy. In addition, the (v, 0) bands with v=6-10 of the [14.6](2)Delta(5/2)-Chi (2)Delta(5/2) transition were found to be perturbed by the [15.7](2)Phi(5/2) state. The interaction between the [14.6](2)Delta(5/2) and the [15.7](2)Phi(5/2) states is evident in the progressive increase in hyperfine width of rotational lines of the [14.6](2)Delta(5/2)-X (2)Delta(5/2) transition as the vibrational quantum number increases. Deperturbation procedures were successfully applied to analyze the interaction between these two states. All observed spectra show partially resolved hyperfine structure, and the hyperfine width decreases rapidly as J increases suggested that the hyperfine structure conforms to the Hund's case a(beta) coupling scheme. Accurate molecular and hyperfine constants for the [14.0](2)Phi(7/2), the [14.6](2)Delta(5/2) and the [15.7](2)Phi(5/2) states were obtained and analyzed.

Probing free jet expansions of supercritical fluids

Attempting to improve the comprehension of supersonic molecular beams at elevated pressures we present a comparative study of thermodynamic descriptions of the terminal flow velocity in free jet expansions. As model system we choose carbon dioxide due to its widespread utilization in supercritical fluid technology. Numerical results for the thermodynamic quantities are obtained using a high accuracy equation of state explicit in the Helmholtz free energy. The influence of pressure and temperature on the beam velocity is investigated for a broad range of stagnation conditions. A consistent physical picture is obtained for calculations employing the initial and final molar enthalpies, while enormous discrepancies are found for descriptions based on the molar isobaric heat capacity or the heat capacity ratio. The deviations are particularly pronounced at the gas–liquid phase transition and in the vicinity of the critical point and can be related to the diverse assumptions of ideal gas behavior. It is shown that computations using real fluid enthalpies permit to assess the fraction of condensation in supersonic jets.

State-to-State Vibrational Energy Transfer in OH A2Σ+ with N2

Vibrational energy transfer from v' = 0 to v' = 1 in the excited OH A2Sigma+ electronic state is investigated in the collisional region of a free-jet expansion. Laser excitation is used to prepare high rotational levels in OH A2Sigma+ (v' = 0, N' = 14-22), which lie above the energetic threshold for OH A2Sigma+ (v' = 1). Subsequent collisions with N2 result in population of a distribution of OH A2Sigma+ (v' = 1) product rotational levels that is characterized through dispersed fluorescence spectra. The majority of products are found in the most near-resonant rotational level of v' = 1, with population falling off exponentially in lower rotational levels. Additionally, the efficiency of vibrational energy transfer is determined by comparing the emission from v' = 1 products with that from the initially prepared v' = 0 level. The fractional transfer decreases by an order of magnitude from the highest to lowest initial rotational levels investigated. This decrease is correlated with an increasingly large change in rotational angular momentum between the initial and final states. The results show that angular momentum constraints are the dominant factor in the efficiency of OH A2Sigma+ v' = 0 to v' = 1 vibrational energy transfer at low collision energies.

Rotational Contour Analysis of Jet-Cooled Methyl Hydroperoxide Action Spectra in the Region of the 2νOH and 3νOH Bands

State-selected photodissociation is used to record the partially rotationally resolved action spectra of CH(3)OOH in the region of its first and second OH-stretching overtones (2nu(OH) and 3nu(OH)) under free-jet expansion conditions. From an analysis of the rotational band contours for the OH-stretching states and their corresponding COOH torsion combination bands, effective rotational constants and transition dipole moment orientations are determined for the vibrational eigenstates. The level splitting between the lowest symmetric and antisymmetric pair of COOH torsion levels, 0(+) and 0(-), associated with the 2nu(OH) overtone state is found to be approximately 3.9 cm(-1). Comparison of spectra in the region of the 2nu(OH) and 2nu(OH) + nu(COOH) bands in CH(3)OOH and CD(3)OOH reveals that the spectral features in CH(3)OOH are substantially more perturbed compared to those of its deuterated counterpart, suggesting that modes involving the methyl rotor contribute significantly to promoting intramolecular vibrational energy redistribution (IVR) in CH(3)OOH. Furthermore, a comparison of the average rotational line widths in both CH(3)OOH and CD(3)OOH for the 2nu(OH) and 2nu(OH) + nu(COOH) bands appears to suggest that at these energies, adding one quanta of low-frequency COOH torsional motion does not enhance the IVR rate relative to that of the pure OH-stretching overtone.

Lineshape analysis of stimulated Raman spectra of the near‐nozzle region of free jet expansions

AbstractAn anomalous lineshape of stimulated Raman spectra obtained from the region very close to the nozzle of supersonic pulsed expansions of nitrogen is presented. High‐resolution Raman spectra of the Q branch of the fundamental vibration mode of N2 have been recorded from two different nitrogen expansions at T0 = 295 K and P0 = 1.5–3.5 bar, the lasers crossing the jet axis in the range z/D = 0.25–1.25, where D is the effective nozzle diameter. The combination of Doppler shifts and strong gradients of density and temperature in the near‐nozzle region yield an inhomogeneous broadening and a double peak structure of the recorded Raman line profiles. The comparison of the experimental results with the simulation of the Raman spectrum from this region provides valuable information about the near‐nozzle flow field. The lineshape described here is different from another reported previously in the literature, which is based on a depletion of the density of free molecules on the axis due to condensation. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Laser Induced Fluorescence Spectroscopy of IrN

High resolution laser induced fluorescence spectra of IrN in the spectral region between 394 and 520 nm were recorded using laser vaporization/reaction free jet expansion and laser induced fluorescence spectroscopy. Seven new vibronic transition bands were observed and analyzed. Two = 1 and five = 0 new states were identified. Least squares fit of rotationally resolved transition lines yielded accurate molecular constants for the upper states. Spectra of isotopic molecules were observed, which provided confirmation for the vibrational assignment. Comparison of the observed electronic states of IrB, IrC, and IrN provides a good understanding of the chemical bonding of this group of molecules.

Repulsive and bound parts of the interatomic potentials of the lowest singlet electronic energy states of the MeRg complexes (Me = Zn, Cd; Rg = He, Ne, Ar, Kr, Xe)

Laser-induced fluorescence excitation spectra of MeRg (Me = Zn, Cd; Rg = He, Ne, Ar, Kr, Xe) complexes were recorded using the D 1 Σ 0 + ← X 1 Σ 0 + free ← bound transition. The complexes were produced in their ground state in a free-jet expansion beam and excited with a dye-laser beam directly to the excited state. Analysis of free ← bound unstructured profiles provided a shape of the repulsive part of the D 1 Σ 0 + -state potentials. Valence ab initio calculations of the ZnRg and CdRg ground- and excited-state potentials and electronic transition dipole moments for the studied transition were performed, taking scalar relativistic and spin–orbit effects into account. Results of the calculations show regularities and correlations in the repulsive branches and bound wells of the X 1 Σ 0 + - and D 1 Σ 0 + -state potentials as well as provide information on the bonding character in both electronic energy states. The trends were compared with available experimental results for ZnRg and CdRg as well as for MgRg and HgRg.

A Spectroscopic Study of Nicotine Analogue 2-Phenylpyrrolidine (PPD) Using Resonant Two-Photon Ionization (R2PI), Microwave, and 2D NMR Techniques

Conformational preferences of the nicotine analogue 2-phenylpyrrolidine (PPD) have been studied in both gaseous and solution phases. Theoretical calculations at the MP2 and B3LYP levels point to 5-6 stable conformers which differ in three degrees of conformational freedom; torsion between the two rings, inversion at the pyrrolidine (PY) amine, and PY ring puckering, characterized using the Cremer-Pople definition for pseudorotation. Only one conformer has a trans arrangement between the amino hydrogen and the phenyl substituent. It is 6-8 kJ mol(-1) more stable than the cis conformers, has a perpendicular ring arrangement, and puckers at the nitrogen atom--similar to structures reported for nicotine. Resonant two-photon ionization (R2PI) data, including hole burn spectra, indicate only one conformer is present in the free jet expansion, and band contour analysis suggests assignment to the trans conformer. Confirmation was provided by microwave spectroscopy. Fifty-seven lines measured in the 48-72 GHz region were assigned to 206 b-type transitions and fitted to yield rotational constants within 2 MHz of MP2 values predicted for the trans conformer. The solution-phase conformers of PPD were studied using 1D and 2D (1)H NMR spectroscopy and solvent-based theoretical calculations. In marked contrast to the gas phase, NMR data reveals only cis conformers present in solution. Calculations confirm increased stability for these conformers when placed in simulated chloroform or water environments. Solvent molecules are believed to disrupt a crucial N...H(ortho) stabilizing interaction present within the trans conformer.