Cheng-Chi Chuang

Dynamics of linear and T-shaped Ar–I2 dissociation upon B←X optical excitation: A dispersed fluorescence study of the linear isomer

We report the dispersed fluorescence spectra of the linear and the previously well-studied T-shaped isomers of Ar–I2 following B←X optical excitation for vpump=16–26, below the I2 dissociation limit. The linear isomer has a continuum excitation spectrum. For excitation at the highest pumping energy (vpump=26), the product vibrational state distribution is nearly identical to that observed for excitation above the I2(B) dissociation limit; it shows a broad, nearly Gaussian distribution of I2(B) vibrational states, with about 22% of the available excess energy deposited in translation of the Ar+I2. This gives direct evidence that the “one-atom cage” effect seen above the I2(B) dissociation limit is attributable to the linear Ar–I2 isomer. The product vibrational state distribution becomes increasingly Poisson for decreasing excitation energies, and only about 7% of the excess energy is deposited in translation for vpump=16. The bond energy in the linear isomer is determined from the spectra, 170(±1.5)⩽D0″(l...

Vibrational predissociation dynamics of ArHF (3000) and (3110): Lifetimes and HF product state distributions

The lifetimes and HF (v=2) product rotational distributions (j′=13 to 9) arising from the vibrational predissociation of ArHF (3000) and (3110) states have been determined by laser-induced dispersed fluorescence. The lifetimes of (3000) and (3110) are found to be 3.9(4)×10−6 and 7.3(8)×10−6 s, respectively, showing both intermolecular vibration and HF valence state dependence. The variation in the vibrational predissociation rate for these two states indicates a strong angular dependence of the change of interaction potential with the HF internuclear distance in the complex. The product state distribution of ArHF (v=3) reveals the excess energy, 3380 cm−1, resulting from vibrational predissociation (Δv=−1) is partitioned primarily into HF product rotation. The rotational distribution observed following excitation of the (3110) level is considerably broader than the relatively sharply peaked distribution from (3000). The vHF=3 ground state, (3000), vibrationally predissociates into j′=13 at a rate of τvp=1...

INTERMOLECULAR STATE DEPENDENCE OF THE VIBRATIONAL PREDISSOCIATION OF N2HF

Three new combination bands, at the second overtone of the HF intramolecular stretch, 3ν1, with each of the three low frequency intermolecular modes, have been spectroscopically characterized by intracavity laser‐induced fluorescence for the N2HF complex. The van der Waals stretching, HF and N2 bending frequencies at vHF=3 are determined to be 98.6, 328.6, and 68.5 cm−1, respectively. State dependent vibrational predissociation is observed in these three bands. The variation in the vibrational predissociation rate in these three bands suggests a strong angular dependence of the intermolecular potential with the HF internuclear distance in the complex.

Weak bond stretching for three orientations of Ar–HF at vHF=3

Three new ArHF (vHF=3) states, (3001), (3101), and (3111), have been observed between 11 350 and 11 420 cm−1 by the hot band transitions from (0001) using intracavity laser induced fluorescence. The term values and rotational constants of these levels are: (3001) ν0=11 385.928 98(28) cm−1, B=0.095 546(32) cm−1; (3101) ν0=11 444.258 12(68) cm−1, B=0.090 617(37) cm−1; and (3111) ν0=11 456.076 51(36) cm−1, B=0.091 863(14) cm−1. Observation of the ArHF (3001) state provides the van der Waals stretching frequency for ArHF at v=3, namely 46.8945(4) cm−1=(3001)–(3000). This value shows an increase of 8.208 cm−1 (21%) upon HF v=3←0 valence excitation. The stretching frequency for the T shaped ArHF is (3111)–(3110)=33.7055(5) cm−1. This value is only 7% greater than that observed at v=1. The (vHF101) Σ bend-stretch combination state, corresponding to (νs=1) of the Ar–FH configuration, has not been observed at vHF=0–2. The stretching frequency here is (3101)–(3100)=31.8178(8) cm−1. The soft-mode frequencies reveal ...

Laser-induced fluorescence spectroscopy of Ar2HF at vHF=3: An examination of three-body forces

The vibrational spectrum of Ar2HF in the 11 320–11 430 cm−1 region is recorded by intracavity laser-induced fluorescence. The intramolecular vibrational state, Σ0, in combination with the intermolecular vibrations, assigned as Πin-plane, Πout-of-plane and Σ1, of the complex have been observed. The Σ0 state correlates adiabatically with j=0 of HF (v=3); the Πin-plane, Πout-of-plane, and Σ1 states correlate adiabatically with j=1 of HF (v=3), respectively. We have determined the vibrational band origins (and rotational constants) of ν0=11 323.784 cm−1 (A=0.120 15, B=0.058 30, C=0.038 94 cm−1), ν0=11 387.730 cm−1 (A=0.122 68, B=0.057 05, C=0.038 42 cm−1), ν0=11 426.815 cm−1 (A=0.120 27, B=0.058 15, C=0.038 71 cm−1) and ν0=11 427.400 cm−1 (A=0.120 26, B=0.058 15, C=0.038 71 cm−1) for Σ0, Πin-plane, Πout-of-plane, and Σ1 states, respectively. The vibrational red shift for the pure HF stretch from vHF=0–3 is 49.023 cm−1. The in-plane and out-of-plane bending frequencies are 63.947 and 103.031 cm−1. The Σ1 state...

Spectroscopy of the OC–HF hydrogen-bonded complex at vHF=3

The v(HF)=3 levels of the linear OC-HF complex are observed in the range of 10,800-11,500 cm(-1) using intracavity Ti-sapphire laser-induced fluorescence. The vibrational predissociation linewidths of both (30000) and (3001(1)0) states exceed 5 GHz; thus, the measured spectra are not rotationally resolvable. Under the assumption that these levels are not strongly perturbed, the rotational constants of the two levels are determined to be 0.1100(1) cm(-1) for (30000), 0.1081(1), and 0.1065(1) cm(-1) for f and e sublevels of (3001(1)0), respectively, through band contour fitting. The (30000)<--(00000) band origin is at 10,894.46(1) cm(-1), showing a HF wave number redshift of 478.3 cm(-1). The 4.07 redshift ratio of v(HF)=3 to that of v(HF)=1 indicates a significantly nonlinear increase of the intermolecular interaction energy through HF valence excitation. An ab initio interaction potential surface for HF valence coordinates varying from 0.8 to 1.25 A is used to examine vibrational dynamics. The HF valence vibration v(1) is treated perturbatively, showing that the vibrational redshifts are determined essentially in first order with only a very small second-order contribution. The (3001(1)0)<--(00000) combination transition is observed with the band origin at 11,432.66(1) cm(-1), giving the HF intermolecular bending mode to be 538.2 cm(-1). The high frequency of this vibration, compared to that in similar HF complexes, shows the strong angular anisotropy of the intermolecular interaction potential of OC-HF with respect to the HF subunit. The lifetime of the (3001(1)0) level increases to 28 ps from 14 ps for (30000).

The dependence of intermolecular interactions upon valence coordinate excitation: The υHF=4 levels of ArHF

The valence state dependence of the Ar–HF interaction potential is extended to υHF=4. Three new ArHF (υHF=4) states, (4000), (4100), and (4110), are observed between 14 780 and 14 880 cm−1 using intracavity laser induced fluorescence. The term values and rotational constants of these states are the following: (4000) ν0=14 783.603 23(30) cm−1, B=0.103 606 8(68) cm−1; (4100) ν0=14 867.419 06(70) cm−1, B=0.102 612(27) cm−1; and (4110) ν0=14 875.048 30(39) cm−1, B=0.103 217(19) cm−1, respectively. The spectral red shifts of ArHF (υ000) dramatically increase from 9.654 cm−1 at υ=1 to 48.024 cm−1 at υ=4. The rotational constant of ArHF(4000) increases essentially linearly with HF valence excitation, becoming 1.3% (40 MHz) greater than that observed at υ=0. At υ=4, the outer classical turning point of HF is extended by 0.4 A from re, and there is no evidence for Ar–H repulsion. The spectral red shift for linear hydrogen bonded Ar–HF(υ000) indicates a strong enhancement of binding energy upon HF valence bond exci...

Vibrational predissociation of an inert gas cluster containing an active molecule: The vHF=3 spectrum of Ar3HF

The vHF=3←0 HF valence excitation spectrum of Ar3HF is obtained by intracavity laser induced fluorescence. The spectroscopic constants determined for the vHF=3 level are band origin ν0=11 310.4520(4) cm−1 (corresponding to the vibrational redshift of Δν=−62.355 cm−1), the rotational constants B=0.039 743(5) cm−1 and DJ=2.04(12)×10−7 cm−1. The changes in rotational constants upon HF valence excitation to v=3 are ΔB=1.08(5)×10−4 cm−1 and ΔC=1.01(6)×10−4 cm−1. The increase in C is interpreted as the occurrence of a 0.2% decrease in the Ar–Ar separation. Appreciable line broadening is observed in the Ar3HF (v=3) spectrum. Twenty-three lines in P and R branches are fitted by the Voigt profile with 60 (10) MHz Lorentzian component establishing the occurrence of significant vibrational predissociation for Ar3HF at vHF=3. This rate is consistent with the empirical scaling of the vibrational predissociation rate with the frequency redshift, observed for stronger hydrogen bonded complexes of HF. The scaling of the ...

Rovibrational spectra of the N2–HF complex at the vHF=3 level

We report the analyses of the three intermolecular combination bands of the hydrogen-bonded N2-HF complex at vHF=3, observed by molecular beam intracavity laser induced fluorescence. The origin of the HF intermolecular bending combination band, (3001(1)0)<--(00000), is 11 548.45(3) cm(-1), 328.2 cm(-1) higher than that of the (30000)<--(00000) transition with an origin at 11 220.250(1) cm(-1). The average rotational constant of the (3001(1)0) level is 0.103 63(1) cm(-1), a 4.8% reduction from B(30000)=0.109 21(1) cm(-1). Perturbations are observed as line splittings, increased line widths, and reduced peak intensities of a number of lines of the e and f components of (3001(1)0). In addition, the centrifugal distortion coefficients of both components are large, negative, and different. The N2 intermolecular bend transition (30001(1))<--(00000) has an origin at 11 288.706(1) cm(-1), 68.456(2) cm(-1) above that of the (30000)<--(00000) transition. This is the lowest combination state at v(HF)=3 level. It is unperturbed, yielding B(30001(1))=0.110.10(1) cm(-1). The transition to the intermolecular stretching state, (30100)<--(00000), has an origin at 11 318.858(1) cm(-1) with B(30100)=0.105 84(1) cm(-1). Both the (30100) and (30000) levels show an isolated perturbation at J=4. The Lorentzian component of the line widths, which show considerable variation with soft mode, are GammaL(30000)=490(30) MHz, GammaL(30100)=630(30) MHz, GammaL(3001(1)0)=250(30) MHz, and GammaL(30001(1))=500(50) MHz.

Change of geometry by vibrational excitation: The vHF=3 spectrum and structure of HF–CO2

We have observed the vHF=3 levels of HF–CO2 in the region 11 150 to 11 210 cm−1 using intracavity Ti-sapphire laser induced fluorescence. The complex shown to be quasilinear at v1=vHF=0 and 1 becomes a semirigid bent species at v1=3 with the CO2 submolecule oriented at an angle near 40° with respect to the connector of the CO2 and HF centers of mass. Transitions to the K=0 and 2 levels of the vHF=3 (3 000 000) from the (0 000 000) ground state and to K=1 of (3 000 000) from the (0 000 001) level are observed, showing a 198.36(5) cm−1 HF vibrational redshift. The rotational constants of the (3 000 000) state are A=2.96(2) cm−1, (B+C)/2=0.0742(10) cm−1, 0.0717(10) and 0.0696(10) cm−1 for the K=0, 1, and 2 levels, respectively, and the centrifugal distortion DK=0.270(5) cm−1, which is large but in agreement with mechanical expectation. The observed transition intensities are a consequence of an appreciable rotation of the inertial axes in the transition. The spectral lines are Lorentzian with ΓL, full width ...