Yen-Chu Hsu

Rotational analysis of à 2A1↔X̃ 2E electronic transition of the jet‐cooled methylthio radical

A set of molecular parameters describing both the X and A states of CH3S, has been obtained by a joint fitting of the rotationally resolved electronic transitions observed in a free‐jet‐cooled laser‐induced fluorescence study of CH3S and an earlier microwave study of its X state. The present work shows that because of incomplete information, nearly all of the previously reported molecular parameters for CH3S must be significantly revised. The present observations show an unusual electronic structure for the radical, characterized by a short C–S bond distance and peculiar methyl group geometry in the ground state. The C–S bond is observed to lengthen markedly in the excited A state.

Laser‐induced fluorescence spectroscopy of CCH (X̃ 2Σ+) in vibrationally excited levels up to 4500 cm−1

Vibrationally excited levels of the CCH radical in the X 2Σ+ state up to 4500 cm−1 above the ground vibrational level have been studied both at high pressure and in a supersonic jet by laser‐induced fluorescence (LIF). The CCH radical was produced by photolyzing acetylene with 193 nm laser light. Nineteen UV bands are rotationally analyzed and assigned as transitions of parallel type from X(0,υl2,υ3), (υ2=0–10, υ3=0–2, l=0–3) to four K sublevels (K=0–3) of a common vibrational level, T, of an upper electronic state most likely B 2A′. Among them, bands at 37 010 and 36 075 cm−1 were previously reported [J. Chem. Phys. 98, 6690 (1993)]; an improved spin–orbit constant of X(0,71,0) was determined in this work. Vibrational energies of the l=0 and 1 levels were determined in a simultaneous analysis of UV and IR bands. Assignment of a vibrational level T as the common upper state of the observed LIF bands permits us to determine spectroscopic parameters of the l=2 and 3 levels of the X state, which are rep...

The low‐lying bending vibrational levels of the CCH (X̃ 2Σ+) radical studied by laser‐induced fluorescence

The uv spectrum of the CCH radical was recorded using the laser‐induced fluorescence technique on the 193 nm photolysis product of acetylene. Four 2Π–2Π bands at 38 805, 37 946, 37 010, and 36 075 cm−1 of CCH were rotationally analyzed and assigned as transitions from the (0,v21,0) (v2=1, 3, 5, 7) vibrational levels of the X 2Σ+ state to a common upper vibronic state (denoted as U), which possibly belongs to the 2 2Π state. A simultaneous nonlinear least squares fit of the uv bands, in combination with the infrared transitions previously observed in the X 2Σ+ state, provided improved spectroscopic parameters for the U state and the (0,31,0), (0,51,0), and (0,71,0) levels of the X state.

Vibrational and rotational structure and excited-state dynamics of pyrene

Vibrational level structure in the S(0) (1)A(g) and S(1) (1)B(3u) states of pyrene was investigated through analysis of fluorescence excitation spectra and dispersed fluorescence spectra for single vibronic level excitation in a supersonic jet and through referring to the results of ab initio theoretical calculation. The vibrational energies are very similar in the both states. We found broad spectral feature in the dispersed fluorescence spectrum for single vibronic level excitation with an excess energy of 730 cm(-1). This indicates that intramolecular vibrational redistribution efficiently occurs at small amounts of excess energy in the S(1) (1)B(3u) state of pyrene. We have also observed a rotationally resolved ultrahigh-resolution spectrum of the 0(0) (0) band. Rotational constants have been determined and it has been shown that the pyrene molecule is planar in both the S(0) and S(1) states, and that its geometrical structure does not change significantly upon electronic excitation. Broadening of rotational lines with the magnetic field by the Zeeman splitting of M(J) levels was very small, indicating that intersystem crossing to the triplet state is minimal. The long fluorescence lifetime indicates that internal conversion to the S(0) state is also slow. We conclude that the similarity of pyrenes molecular structure and potential energy curve in its S(0) and S(1) states is the main cause of the slow radiationless transitions.

193.3 nm photodissociation of acetylene: Nascent state distribution of CCH radical studied by laser‐induced fluorescence

The nascent rovibronic distribution of CCH radicals in the 193.3 nm photolysis of acetylene has been measured by laser‐induced fluorescence in a supersonic jet. CCH fragments in the X 2Σ+ state are vibrationally hot, but rotationally cold. Predominant CCH fragments were observed at levels of the X state with large mixing of A‐state character, particularly levels near the potential minimum of A 2Π. This indicates that a nonadiabatic transition near the exit channels plays an important role in the 193.3 nm photodissociation of acetylene. Some, but not all, of the K=1 levels have distinctively bimodal rotational distributions. The relative vibrational energy distributions obtained from this work were used to simulate the translational energy distribution of the hydrogen atom by Balko, Zhang, and Lee [J. Chem. Phys. 94, 7958 (1991)] to extract the population distribution of CCH. It is thus determined that the majority of CCH radicals are formed in the ground electronic state (X). Less than half of the CCH ...

The 4051-Å band of C3 (ÃΠu1−X̃Σg+1, 000-000): Perturbed low-J lines and lifetime measurements

Rotational analyses have been carried out at high resolution for the 000-000 and 000-100 bands of the A (1)Pi(u)-X (1)Sigma(g) (+) transition of supersonic jet-cooled C(3). Two different spectra have been recorded for each band, using time gatings of 20-150 and 800-2300 ns. At the shorter time delay the spectra show only the lines observed by many previous workers. At the longer time delay many extra lines appear, some of which have been observed previously by [McCall et al.Chem. Phys. Lett. 374, 583 (2003)] in cavity ring-down spectra of jet-cooled C(3). Detailed analysis of these extra lines shows that at least two long-lived states perturb the A (1)Pi(u), 000 state. One of these appears to be a (3)Sigma(u) (-) vibronic state, which may possibly be a high vibrational level of the b (3)Pi(g) state, and the other appears to be a P = 1 state with a low rotational constant B. Our spectra also confirm the reassignment by McCall et al. of the R(0) line of the 000-000 band, which is consistent with the spectra recorded towards a number of stars that indicate the presence of C(3) in the interstellar medium. Fluorescence lifetimes have been measured for a number of upper-state rotational levels. The rotational levels of the A (1)Pi(u) state have lifetimes in the range of 230-190 ns, decreasing slightly with J; the levels of the perturbing states have much longer lifetimes, with some of them showing biexponential decays. An improved value has been obtained for the nu(1) vibrational frequency of the ground state, nu(1) = 1224.4933 +/- 0.0029 cm(-1).

Laser spectroscopy of CCH in the 36 600–39 700 cm−1 region

This work reports on nine new bands of the CCH B 2A′←X 2Σ+ system recorded by laser-induced fluorescence. In addition, the 37 946 and 38 107 cm−1 bands were revisited in a supersonic molecular beam. Of these 11 bands, ten were in the 36 600–39 700 cm−1 range; they were rotationally analyzed and assigned as transitions from low vibronic levels, X(0,v2,v3) (v2=2–4 and v3=0–1) of CCH to two vibrational levels (T,T+1221) of the B state. Although previously observed as too weak, these bands were enhanced in this work by relaxing vibrationally excited CCH with SF6, instead of He or Ar. The rotationally resolved 35 939 cm−1 band, the band outside of the range of 36 600–39 700 cm−1, is assigned as B(T+775)←X(0,91,0). These new bands, together with transitions previously reported, are simultaneously analyzed in a nonlinear fit. This work obtained the first spectroscopic parameters of X(0,20,0), X(0,22,0), X(0,22,1), B(T+775), and B(T+1221), and further improved the spectroscopic parameters of X(0,31,0)...

Nascent metastable products in the reactions Ba+NO2 and Ba+N2O

The chemiluminescent reactions Ba+N2O and Ba+NO2 are studied using laser‐induced fluorescence under molecular beam and low pressure flow conditions. The studies indicate the nascent electronic branching between chemiluminescent and metastable levels for the two reactions. In both reactions the predominant product channel is the ground electronic state. For the more exothermic Ba+N2O reaction, the exothermicity is mainly deposited into vibrational and rotational excitation of ground electronic state levels which exceeds the energies of the first few excited electronic states of BaO. Excited electronic states which are populated are also vibrationally and rotationally hot. For the Ba+NO2 reaction the predominant ground state products are vibrationally cold, but rotationally hot. Excited states produced are both vibrationally and rotationally cold. Selective production of particular metastable electronic states is seen.

The multiphoton dissociation of acetylene. I, Long-lived intermediates, sequential and concerted processes of dissociation

The multiphoton dissociation processes of acetylene via a two‐photon resonant predissociative state, v=0 of 1Σ+g, have been studied by three techniques: time‐resolved photofragment excitation spectroscopy (TRPFES), laser‐induced fluorescence (LIF) of the C2 fragments, and dispersed emission. We found that the major dissociation products are H atoms, H2 molecules, and C2 molecules in the X 1Σ+g, a 3Πu and A 1Πu states; among the latter, C2 X 1Σ+g molecules are formed by a sequential bond–rupture mechanism whereas some C2 in a 3Πu is formed by a concerted two‐bond fission process. Other, minor dissociation channels due to three‐photon processes, such as C2(d 3Πg)+2H(2S(1)/(2)), C2(d 3Πg)+H2(X 1Σ+g), C2(C 1Πg)+H2(X), C2(e 3Πg)+H2(X), and C2(D 1Σ+u)+H2(X), were also detected. In the 2+1 concerted dissociation yielding C2(C 1Πg)+H2(X), a long‐lived intermediate C2H2, likely a cis isomer or other conformer in which the hydrogen atoms are relatively close to each other, was revealed by TRPFES; its zero‐pressure ...

Ultraviolet laser-induced fluorescence of the C2H radical

Abstract In the spectral range 34500–40000 cm −1 , two molecular species were detected by means of laser-induced fluorescence in the 193 nm photolysis of acetylene: one is C 2 (a 3 Π u ), the other is most likely C 2 H( X ). The identification of C 2 H is based u the rotational constants ( B ″ = 1.4397–1.4534 cm −1 ), the deuterium isotope effect on the vibronic bands, and the wavelength-resolved fluorescence.