Supersonic Planar Jet Research Articles

Numerical Investigation of Hydrogen Strut Injections into Supersonic Airflows

The ine uence of different injector geometries on the mixing behavior of planar supersonic jets is investigated. Hydrogenisinjectedinthee owdirectionthroughthebluntendofastrut.Differentlipthicknessesattheinjectorend induce different extents of recirculation zones. Additionally, changes are caused within the shock-wave/expansion fan pattern at the injector exit that have an important ine uence on loss in total pressure. The accuracy of the used numerical scheme is demonstrated for one injector geometry by comparison with experimental data. Based on this cone guration, modie cations in lip thickness and injection Mach number are investigated numerically and are assessed using several calculated performance parameters. Results show that the chosen lip thickness has a much stronger ine uence on loss in total pressure than moderate modie cations in injector length and height. For the very thin hydrogen jets that were investigated, the mixing efe ciency is nearly independent on injector lip thickness.

The mystery of the CO dimer: assignments from variable-temperature jet-cooled infrared spectra

The infrared absorption spectrum of the CO dimer was studied in the vicinity of the CO fundamental band in both axisymmetric and planar supersonic jet expansions. By probing the jet at varying distances downstream, we observed (CO) 2 spectra over a range of effective rotational temperatures from about 15 down to 1 K. The temperature dependence of the transitions enabled us to identify one perpendicular and two parallel subbands arising from the lowest rotational levels of the dimer. The effective intermolecular separation for (CO) 2 was determined to be about 4.35 Å. The only previous assignment of CO dimer spectra is incompatible with our results.

Free shear layer interaction with an expansion-compression wave pair

Experiments were performed using high-speed cinematography to spatially and temporally resolve compressible planar mixing layer structures in pressure matched and unmatched conditions. The unmatched conditions (overexpanded and underexpanded) were achieved by enforcing a static pressure difference between the supersonic planar jet exit (M ∼ 1.65) and the ambient quiescent flow below. The pressure mismatch resulted in the interaction of expansion-compression waves with the free shear layer. Temporally resolved planar images, autocorrelations, and temporal correlations were obtained for a portion of the flowfield just downstream of the expansion-compression interaction. The results showed increased fluctuations in shear layer structure angles for the unmatched pressure conditions as compared with those for the matched condition. The degree of angular fluctuations qualitatively correlated with a nondimensional pressure gradient parameter, and the time scale was consistent with the eddy passage frequency. The passive scalar convection speeds for the unmatched conditions were found to be significantly lower than for the matched cases.

A near-infrared Fourier-transform planar supersonic jet spectrometer for rovibrational studies of weakly bound dimers and trimers

A high-resolution FTIR supersonic slit jet absorption spectrometer is described for the investigation of weakly bound dimers and trimers in the near-infrared spectral region. The spectrometer is demonstrated to conservatively have a sensitivity of 6×108 molecules/cc/state and can be operated at an apodized resolution of 0.004 cm−1. To illustrate the performance of the spectrometer, it has been applied to the rovibrational analysis of the band spectra of three weakly bound species in the near infrared: ν1 OC–HF, ν1 N2–HCl, and ν5 (H35Cl)3. The recorded spectra of these species are compared with corresponding investigations using state-of-the-art tunable infrared laser supersonic jet or molecular-beam spectrometers to illustrate the capabilities and limitations of the current FTIR supersonic jet spectrometer.

Far-infrared vibration-rotation-tunnelling spectroscopy of ArDCl

Three intermolecular vibrations of the ArDCl complex have been observed by tunable far-infrared laser spectroscopy in a supersonic planar jet. Vibration-rotation-tunnelling transitions to states correlating to the j = 1, Θ = 0, the j = 2, Θ = 0, and the j = 2, Θ = 1 internal rotor levels of DCl have been measured for both chlorine isotopes with nuclear quadrupole hyperfine resolution. The fitted spectroscopic constants are compared with recent calculations from the new H6(4, 3, 0) ArHCl potential by Hutson and it is concluded that, although the new potential is very accurate, significant discrepancies between observed and calculated values exist for states which probe the secondary minimum (ArClH) of the potential.

A cw planar jet computer-controlled tunable IR diode laser spectrometer for the investigation of hydrogen-bonded complexes

An ultrasensitive computer-controlled tunable infrared diode laser cw planar supersonic jet spectrometer for second derivative detection of hydrogen-bonded complexes is described. Spectroscopic analyses of transitions in carbon monoxide monomer permit detection limits to be set at 2.9×106 molecules/cm3/quantum state. Simultaneously recorded spectra of the ν2C≡O stretching vibrations in OC—HCl and OC—HBr are presented to illustrate the performance of this supersonic jet spectrometer for investigating weakly bound hydrogen-bonded dimers.

Tunable far-infrared laser spectroscopy of van der Waals bonds: thejkc= 10← 00Σ bending vibration of Ar-14NH3

Hyperfine resolved spectra have been measured for the Ar-NH3 complex over the range 21–28 cm-1 using a tunable far-infrared laser in combination with a continuous planar supersonic jet. Twenty-three transitions are assigned to the lowest Σ-bending vibration (v 0 = 26·470633(77)cm-1) from the (asymmetric) inversion level of the Σ, j k c = 00 internal rotor state of NH3 to the Σ, (10) symmetric inversion level. A nearly free-rotor model is used to deduce the zeroth-order intermolecular vibrational energy-level diagram and corresponding selection rules, which are used in assigning the spectra. All evidence obtained in this study supports the contention of Nelson et al. that NH3 is an inverting nearly free rotor in this T-shaped complex.

The submillimeter rotation tunneling spectrum of (D 2O) 2

By direct absorption of submillimeter radiation in a planar supersonic jet, we have measured K a =1→ K a =2 transitions of the fully deuterated water dimer (D 2O) 2 in the region between 350 and 430 GHz. Several previously unknown constants have been derived. The A-rotational constant is determined as 120327.492(32) MHz. The sum or the difference of the largest tunneling splittings for K a =1 and K a =2 is approximately 21 GHz. The tunneling matrix element h 2 v has been found to be a factor of 100 smaller than in (H 2O) 2

The rovibrationally resolved spectrum of ν 2 OCHBr

A band analysis of the rovibrationally resolved spectrum of a hydrogen-bonded complex involving the proton donor HBr is reported. The ν 2 (CO stretching vibration) in OCHBr has been observed using a computer-controlled cw supersonic planar jet diode laser spectrometer. The following fitted molecular parameters have been determined (in cm −1) for the ν 2 band of 16O 12C 1H 79Br: ν 0 = 2152.605023(60), B 2 = 0.03820684(15), D J (2) = 9.201(8) × 10 −8, B 0 = 0.03837695(1), D J (0) = 9.016(1) × 10 −8; and for the ν 2 band of 16O 12C 1H 81Br: ν 0 = 2152.606257(59), B 2 = 0.03795714(77), D J (2) = 8.872(32) × 10 −8, B 0 = 0.03812694(81), D J (0) = 8.743(34) × 10 −8. Investigation of the observed line profiles has allowed a lower limit of 0.55 ns to be placed on the excited state predissociative lifetime.

Direct measurement of the HCl dimer tunneling rate and Cl isotope dependence by far-infrared laser sideband spectroscopy of planar supersonic jets

The large amplitude tunneling motion of the HCl dimer has been directly studied with a tunable far-infrared laser sideband/two-dimensional free jet expansion spectrometer at hyperfine resolution. Rotationless tunneling rates for the three common chlorine isotopic forms are v(35–35) =463 979.2(1) MHz, v(35–37)=463 357.7(1) MHz, and v(37–37)=462 733.7(3) MHz. Both the rotational constants and hyperfine parameters indicate that the vibrationally averaged structure shows little variation within a given tunneling state, with both HCl bond angles giving an average projection on the a-axis of 47° in all states with resolved hyperfine patterns.

Microwave direct absorption spectroscopy of weakly-bound clusters in a planar supersonic jet: Spectra of Ar-HCN and (HCN) 2 from 17.4 to 76.7 GHz

A direct absorption microwave and millimeterwave spectrometer capable of observing the rotational transitions of weakly-bound complexes formed in a planar supersonic nozzle has been constructed. Both the technique for observing weakly-bound complexes and its initial application to Ar-HCN and (HCN) 2 are described. The Ar-HCN lines reported here show significant frequency discrepancies (up to −3187.2 MHz) from predictions using previously obtained constants with distortion treatments through sixth order in J, reflecting the strong angular-radial coupling and pervasive intermolecular vibrational state mixing in this complex.

Measurement of the perpendicular rotation-tunneling spectrum of the water dimer by tunable far infrared laser spectroscopy in a planar supersonic jet

Fifty-six transitions from the K=1 lower→K=2 lower tunneling–rotation band of water dimer have been measured and assigned at 22 cm−1 by direct absorption spectroscopy in a cw planar supersonic jet expansion using a tunable far infrared laser spectrometer. Two different models were used to fit the data and several spectroscopic constants were determined for the upper and lower states. This work supports the local IAM model recently proposed by Coudert and Hougen for the hydrogen bond tunneling dynamics of the water dimer. This model includes four different tunneling motions, all of which contribute to the observed tunneling splittings. This is the most complicated hydrogen bonded system considered to be well understood at this time, at least in the lowest few K states.

ChemInform Abstract: Excess Energy Dependence of Internal Conversion in the S1 State of Azulene.

The internal conversion (IC) rates for nine single vibronic levels of the S1 state of azulene, having from 0 to 2417 cm−1 of vibrational energy, have been measured by direct absorption spectroscopy in a pulsed, planar supersonic jet. The value of kIC at the vibrationless level is 9.4×1011 s−1 , with an approximately linear increase to 3.3×1012 s−1 at the highest level studied. This excess energy dependence of the rate constant is contrasted to that for S2 →S1 IC across an almost identical electronic energy gap, and an explanation involving low‐frequency modes is offered.

Tunable far infrared laser spectroscopy of van der Waals bonds: Vibration–rotation–tunneling spectra of Ar–H2O

The first high resolution spectra of a rare gas–H2O cluster have been observed using a tunable far infrared laser to probe the vibration–rotation–tunneling levels of Ar–H2O formed in a continuous planar supersonic jet. The high sensitivity of this spectrometer facilitated extensive measurements of two perpendicular subbands which are assigned to transitions from the ground state to the upper component of a hydrogen exchange tunneling doublet (c-type) at 21 cm−1, and to vb1 =1+ (b-type) at 25 cm−1, the lower tunneling component of a bending vibration which is perpendicular to the tunneling coordinate. The tunneling splitting is shown to be in the range 2.5–7 cm−1 and the lower tunneling component of the excited bending vibration lies between 39 and 43 cm−1 above the ground state of the complex. The experimentally determined center-of-mass separation (Rc.m. =3.75 Å) and harmonic stretching force constant (ks =0.0134 mdyn/Å) are compared to those of related first and second row hydrides. The large amplitude motions occurring within this complex make it difficult to establish its structure.

Excess energy dependence of internal conversion in the S1 state of azulene

The internal conversion (IC) rates for nine single vibronic levels of the S1 state of azulene, having from 0 to 2417 cm−1 of vibrational energy, have been measured by direct absorption spectroscopy in a pulsed, planar supersonic jet. The value of kIC at the vibrationless level is 9.4×1011 s−1 , with an approximately linear increase to 3.3×1012 s−1 at the highest level studied. This excess energy dependence of the rate constant is contrasted to that for S2 →S1 IC across an almost identical electronic energy gap, and an explanation involving low-frequency modes is offered.

Energy-resolved phosphorescence of the lowest triplet state of pyrazine in a pulsed planar supersonic jet

Energy-resolved phosphorescence of the lowest triplet state of pyrazine in a pulsed planar supersonic jet

Internal conversion rates for single vibronic levels of S2 in azulene

Simultaneous measurements of absorption and fluorescence in a pulsed planar supersonic jet are used to determine fluorescence quantum yields of nine single vibronic levels (SVL’s) in the S2 electronic manifold of azulene. The quantum yield at the spectroscopic origin is 0.042±0.004, somewhat larger than that obtained in earlier room-temperature measurements in solution. Most SVL’s have quantum yields within experimental error of this value, except for two SVL’s characterized by large-amplitude motion at carbons 1 and 3, whose quantum yields are about 0.06. Our quantum yield data are combined with recent lifetime measurements by other workers to obtain fluorescence and internal conversion decay rates for each SVL. The radiative rates vary by a factor of 2, as expected for the S2↔S0 transition, whose absorption strength is strongly enhanced by vibronic coupling between S2 and S4. The internal conversion rates increase with excess vibrational energy in a way that parallels the extent of intramolecular vibrational redistribution (IVR), as determined by other workers using SVL emission and time-and-energy-resolved emission. Thus, the internal conversion rates in this electronic state appear to exhibit behavior typical of large aromatic molecules.

Optical selection studies of electronic relaxation from the S1 state of jet-cooled anthracene derivatives

In this paper we explore the energy dependence of the interstate electronic relaxation rates knr from the S1 manifold of anthracene and seven of its isotopic and chemical derivatives, which were inferred from quantum yield data. Absolute fluorescence quantum yields Y from groups of rotational states within the electronic origin S1(0) and from vibrational states were obtained over the excess energy Ev=0–3000 cm−1 above S1(0) by the simultaneous interrogation of the fluorescence excitation spectra and of the absorption spectra in seeded, pulsed, planar supersonic jets of Ar. Additional information was obtained from quantum yield data of van der Waals (vdW) complexes of these molecules with Ar. The fluorescence quantum yields from the S1(0) of anthracene, 9-cyano-anthracene, and 9,10-dibromoanthracene were found to be independent of the rotational state, providing further evidence for the rotational independence of knr from a single doorway state. From the Y data of the electronic origins and from the Ev dependence of Y we conclude that intersystem crossing (ISC) dynamics of the S1 manifold is dominated by the interplay between two classes of nonreactive coupling and/or relaxation: (i) Interstate coupling, involving the superposition of direct S1→(T1) ISC together with S1→(Tx) →(T1) mediated ISC through a sparse or dense (Tx) manifold of a higher triplet state; (ii) Intrastate coupling within the S1 manifold, which sets in with increasing Ev and which results in intramolecular vibrational energy redistribution (IVR) at high Ev. The dominant role of mediated interstate coupling in ISC dynamics from S1(0) and from low Ev states was inferred from the inverse deuterium isotope effect on the ISC rates, the extreme sensitivity of knr of deuterated anthracene to a single H atom substitution, and to level shifts induced by complexing with Ar, as well as from the three-orders-of-magnitude difference between the knr values from the S1(0) of 9-bromoanthracene and of 9,10-dibromoanthracene. The onset of the mediated ISC is documented by an abrupt drop of Y in the narrow (Ev =617–805 cm−1) energy range for 9,10-dichloroanthracene and by the oscillatory energy dependence of Y vs Ev and the extreme energy sensitivity of Y in the range Ev =157–800 cm−1 of 9,10-dibromoanthracene, which is attributed to near-degeneracies between S1 states and the mediating (Tx) states. These resonance effects can be drastically modified by dispersive level shifts induced by complexing with Ar. At high excess vibrational energies some universal features of the Ev dependence of knr are exhibited. These involve a gradual increase of k n r with increasing Ev at medium energies (Ev =1000–1800 cm−1), which correspond to the intermediate level structure for intrastate coupling and a very weak Ev dependence of knr at high energies (Ev =1800–3000 cm−1), which manifest the effect of statistical intrastate IVR on interstate ISC.

Spectroscopy and dynamics of 9,10-dichloroanthracene-Ar n van der Waals complexes

In this paper we report on the measurements of the absorption spectra of large van der Waals complexes in planar supersonic jets. The absorption spectra and the fluorescence excitation spectra of the complexes of 9,10-dichloroanthracene (DCA) with Ar atoms are reported for the S 0 → S 1(0) vibrationless transition of DCA·Ar n ( n = 1−6), and from the S 0 → S 1 (1390 cm −1) transition of DCA·Ar n ( n = 1−4). Information on the structure of these complexes was inferred from the additivity of the spectral shifts per added rare-gas atom (ASSRA) for DCA·AR n ( n = 1.2) and from deviations from the ASSRA for CDA-Ar n ( n = 3−6). The vibrational predissociation (VP) dynamics of DCA-Ar n ( n = 1−3) complexes was interrogated by fluorescence quantum yield, Y, measurements. The value of the S 1 (1390 cm −1) state of DCA·Ar n ( n = 1−3) exhibits a dramatic enhancement relative to that of DCA. Utilizing the dependence of Y on the excess vibrational energy of bare DCA, we were able to estimate the internal energy of the fragments resulting from VP of DCA·Ar n of DCA·Ar n ( n = 1−3). An upper limit of ⋍ 100 ps was estimated for the VP (and/or vibrational energy redistribution) lifetime from the S 1 (1390 cm −1) state of DCA·Ar 3.

Inverse deuterium isotope effect on radiationless electronic transitions

Fluorescence quantum yields, Y, front photoselected vibrational states in the S 1 manifold of anthracene and of perdeuterated anthracene in planar supersonic jets reveal a large inverse deuterium isotope effect on the non-radiative relaxation from the S 1 origin ( Y H/ Y D = 5), while for high (⩾1500 cm −1) excess energies above the S 1 origin the inverse isotope effect is eroded ( Y H/ Y D ≈ 1). Novel information emerges on intermediating states involved in intersystem crossing.