Excitation Spectrum of P-Dimethoxybenzene Excitation LIF, (Vibronic) Spectrum Contains Both Rotamers

Vibronic Spectroscopy In a given electronic state, a polyatomic molecule may be excited to any of the possible vibrational level. Each of these levels may be called vibrational-electronic state, or, for short, a vibronic state. Vibronic energy As long as the electronic state is non-degenerate, the vibronic

energy (Eev) is simply the sum of the electronic energy (Ee) and

the vibrational energy (Ev).

Eev = Ee + Ev Vibronic eigenfunction and vibronic species

In a first approximation the vibronic eigenfunction (ψev) describes the electronic and vibrational motions.

wbt 1 Vibronic eigenfunction

ψev = ψe (q, 0) ψv (Q)

where ψe (q,0) is the electronic eigenfunction for the equilibrium position and ψv (Q) is the vibrational eigenfunction. Note that q and Q stand for all the electronic and nuclear (normal)

coordinates, respectively. Both ψe and ψv have symmetry properties in accord with one of the species of the point group of

the equilibrium configuration. One may apply the supersonic jet or molecular beam technique

to prepare a molecule in the electronically ground (S0) state. Then, a vibronic spectrum can be recorded by using the excitation laser-induced fluorescence (LIF) or resonance-

enhanced multiphoton ionization (REMPI) spectroscopy. wbt 2 wbt 3 Excitation laser-induced fluorescence (LIF) Spectroscopy Excitation laser-induced fluorescence (LIF) is spontaneous emission from molecules that have been excited by laser radiation. If a molecule resonantly absorbs an UV photon from the laser beam, the molecule is left in an electronically excited (or vibronic) state. Such a state is unstable and will decay spontaneously, emitting a photon again. The molecule in the excited state of finite lifetime (10-6‒10-8 s) emits its photon on return to a lower energy level in random (spherically) directions. In most experiments, the emitted fluorescence is collected at 90° to a collimated laser beam (which is usually set at 90° to a supersonically expanded jet or a collimated molecular beam). In a 3-dimensionally view, the molecular beam, laser beam and fluorescence detector are perpendicular to each others. wbt 4 Ref.: Smalley, et al. J. Chem. Phys. 64, 3266 (1976). wbt 5 wbt 6 Excitation LIF (vibronic) spectrum of toluene

-1 1 461 cm , 6a 0 -1 1 530 cm , 6b 0 -1 1 753 cm , 1 0 -1 1 933 cm , 12 0 -1 1 965 cm , 18a 0

Electronic excitation energy = 37477 cm-1 0 0 The spectral feature is called the 00 band, indicating toluene is at the S10 state. Smalley, J. Chem. Phys. 72 (1980) 5039-5048. wbt 7 苯環的30種振動模式 Tangential vibrations： (1) carbon-carbon stretching (2) C-H(X) in-plane bending modes

Radial vibrations： (1) C-H(X) stretching vibrations (2) radial skeletal vibrations

Out-of plane vibrations： (1) out-of plane skeletal vibrations (2) C-H(X) out-of-plane vibrations

Laser-induced fluorescience (LIF) excitation spectrum of p-dimethoxybenzene Excitation LIF, (vibronic) spectrum contains both rotamers

Hole-buning LIF, (vibronic) spectrum of trans rotamer

Hole-buning LIF, (vibronic) spectrum of cis rotamer

Patawi, et al. Chem. Phys. Lett. 316 (2000) 433-441. IVR in the S1 state of wbt jet-cooled cis- and trans-p-dimethoxybenzene 9 In the present case, the cis and trans rotational isomers (rotamers) of p- dimethoxybenzene coexist in the chemical sample. The obtained vibronic spectrum include both rotamers, as seen in Fig 1(a). One may apply spectral hole-burning spectroscopy to distinguish the two rotamers. Fig 1 (b) and 1(c) show the vibronic spectra of cis- and trans-p- dimethoxybenzene, respectively. For the spectral hole-burning experiments, two tunable UV lasers are used: 0 (a) pump (burn) laser: pulse energy ~500 µJ [fix laser frequency at the 0 0 band of the unwanted rotamer] (b) probe (scanning) laser: pulse energy ~20 µJ [scan laser to cover the range of vibronic transition to obtain the excitation spectrum of the other (selected) rotamer] (c) Time delay of the pump and probe lasers is about 100 ns. (d) Total fluorescence is detect by a photomultiplier (PMT).

wbt 10 One-color resonant two-photon ionization (1C-R2PI)

11 Photoionization efficiency (PIE) curve By 2C-R2PI

Ionization: Laser 2 freq. scanned

Excitation: Laser 1 freq. fixed (species selection) 12 Mass-analyzed threshold ionization (MATI)

PFI V+ -1 after IE = 64195 cm V+ 10 μs V+

Excitation: -

ΔE = 30930 cm-1 cm Laser 2 / Wavenumber Relative freq. Ion intensity scanned S1

Excitation: Laser 1 ΔE = 33265 cm-1 freq. fixed species and 3,4-difluoroanisole state S0 selection 13 1C-R2PI (vibronic) spectrum of p-dimethoxybenzene

Tzeng, et al. J. Mol. Struct. 448 (1998) 91-100. Structures and vibrations of p-

dimethoxybenzene in the S0 and S1 states studied by ab initio and resonant two- photon ionization spectroscopy

wbt 14 MATI (cation) spectrum of cis-p-dimethoxybenzene

Frequency of the excitation (pump) laser 0 -1 is fixed at S10 (33852 cm ) IE (cis rotamer) = 60772 cm-1

Frequency of the excitation (pump) laser 1 -1 is fixed at S16a (34357 cm ) IE (cis rotamer) = 60772 cm-1

Frequency of the excitation (pump) laser 1 -1 is fixed at S11 (34658 cm ) IE (cis rotamer) = 60772 cm-1

Lin, et al. J. Phys. Chem. A 105 (2001) 11455-11461. Mass-analyzed threshold ionization spectroscopy of the selected rotamers of hydroquinone and p- dimethoxybenzene cations wbt 15 MATI (cation) spectrum of trans-p-dimethoxybenzene

Frequency of the excitation (pump) laser 0 -1 is fixed at S10 (33631 cm ) IE (trans rotamer) = 60563 cm-1

Frequency of the excitation (pump) laser 1 -1 is fixed at S16a (34013 cm ) IE (trans rotamer) = 60563 cm-1

Frequency of the excitation (pump) laser 1 -1 is fixed at S11 (34438 cm ) IE (trans rotamer) = 60563 cm-1

Lin, et al. J. Phys. Chem. A 105 (2001) 11455-11461. Mass-analyzed threshold ionization spectroscopy of the selected rotamers of hydroquinone and p- dimethoxybenzene cations wbt 16 Normal vibration 6a of cis- and trans-p-dimethoxybenzene in

the S1 and D0 states

Possible stable structures of resorcinol

Isomer 1 = structure A [hole-burning] Isomer 2 = structure B [hole-burning] Structure C, not detected 1C-REMPI spectrum contains vibronic features of both rotamers. When one fix frequency of the burn laser at 36196 cm-1 and scan the probe laser, he can obtain the vibronic (hole-burning) spectrum of isomer 1 (structure A) of resorcinol. Fixing frequency at 35944 cm-1, one can obtain the vibronic spectrum of isomer 2 (structure A). Gerhards, et al. Chem. Phys. Lett. 240 (1995) 506-512. Rotamers and wbt vibrations of resorcinol obtained by spectral hole burning 18 Conformers of p-n-propylphenol

trans, 0 cm-1 (B3PW91/6-311++G**) 0 cm-1 (RHF/6-311++G**)

gauche-A, 211 cm-1 (B3PW91/6-311++G**) 285 cm-1 (RHF/6-311++G**)

gauche-B, 214 cm-1 (B3PW91/6-311++G**) 305 cm-1 (RHF/6-311++G**)

wbt 19 1C-R2PI spectrum of p-n-propylphenol

0 0 (t) 0

6a1 g 0 a

1 (O-H) 10a 0 g 1 b 9b 1 0 1 (C-C H ) 1 0 3 7 121

Relative Intensity Relative 4 0 0

35400 35600 35800 36000 36200 36400 One-photon energy / cm-1 wbt 20 Some normal modes of p-n-propylphenol

(C-C H ) 3 7 10b, (C-OH) S , 66 (64) cm-1 S , - (272) cm-1 1 1 D , 64 (66) cm-1 D , 297 (304) cm-1 0 0

6a, (CCC) 1, breathing S , 460 (459) cm-1 S , 810 (807) cm-1 1 1 D , 485 (480) cm-1 D , 829 (832) cm-1 0 0 wbt 21 2C-R2PI (PIE) spectra of p-n-propylphenol rotamers

(a) 65275 via S 00 (35501 cm-1) trans 1

(b) 0 -1 via S10 (35453 cm ) gauche-A 65380

Ion Intensity Ion

65200 65300 65400 65500

-1 wbt Two-Photon Energy / cm 22 MATI spectra of p-n-propylphenol rotamers

+ -1 0 , 65273 cm (a) trans, 0 -1 via S10 (35501 cm )

1 10b 1 1 1 1 1 11 1 9a 8a 9b

+ -1 0 , 65375cm gauche-A (b) 0 -1 via S10 (35453 cm )

1 1 10b 1 1 7a 1 1 9a 8a 111

Relative Intensity Relative

+ -1 0 , 65357 cm (c) gauche-B 0 -1 via S10 (35441 cm ) 1 1 7a1 C-C   1 3  9a 8a1

0 500 1000 1500 1 wbt Ion Internal Energy / cm 23 Energy levels (in cm-1) of p-n-propylphenol rotamers

trans gauche-A gauche-B

D0 8 313 65375 65357 65273

S1 163 12 35441 35501 35453

S0 3 wbt 211 24