Gregory V. Hartland

Time‐resolved Fourier transform spectroscopy with 0.25 cm−1 spectral and <10−7 s time resolution in the visible region

The development of a new time‐resolved Fourier transform spectrometer that is capable of 0.25 cm−1 spectral resolution and better than 10−7 s temporal resolution in the visible is reported. The time‐resolved capability of the spectrometer is achieved by coupling a step‐scan interferometer to a transient digitizer/laser system. The operation of the spectrometer is described in detail, and scattered light and laser‐induced fluorescence spectra from an I2 gas cell are presented to demonstrate the temporal and spectral resolution of the spectrometer.

COLLISIONAL DEACTIVATION OF HIGHLY VIBRATIONALLY EXCITED NO2 MONITORED BY TIME-RESOLVED FOURIER TRANSFORM INFRARED EMISSION SPECTROSCOPY

Infrared emission from highly vibrationally excited NO2, prepared by collision induced internal conversion, can be detected with 1 cm−1 spectral and 0.5 μs time resolution over the 800–10 000 cm−1 range by time‐resolved Fourier transform emission spectroscopy. The energy distribution of vibrationally excited NO2 during collisional deactivation can be extracted from the emission spectra and shows that the energy loss per collision increases dramatically from <50 cm−1 below 13 000 cm−1 energy to 1300 cm−1 at 20 000 cm−1 energy.

Fourier transform dispersed fluorescence spectroscopy: Observation of new vibrational levels in the 5000–8000 cm−1 region of ã 1A1 CH2

Dispersed fluorescence spectra from the CH2 b 1B1 state to highly excited vibrational levels of the a 1A1 state were recorded by a new technique, Fourier transform dispersed fluorescence spectroscopy. The spectra obtained clearly show the advantages of using a Fourier transform spectrometer for dispersing fluorescence, namely emission over a wide spectral range can be efficiently detected with high sensitivity and resolution. These advantages allow four new vibrational levels in the CH2 a 1A1 state to be observed; the (2,0,0) and (0,5,0) vibrational overtones and the (1,2,0) and (1,3,0) combination bands. The vibrational term values for these levels are given, along with the harmonic frequencies and anharmonicities for the v1 and v2 modes. From the (0,5,0) term value an improved estimate of the barrier height to linearity in the CH2 a 1A1 state is made.

Collisional energy transfer of highly vibrationally excited NO2: The role of intramolecular vibronic coupling and the transition dipole coupling mechanism

The collisional relaxation of highly vibrationally excited NO2 has been studied for a variety of collision partners (He, Ar, CO, N2, O2, N2O, NO2, CO2, SF6, and toluene) by time-resolved Fourier transform infrared emission spectroscopy. The average energy 〈E〉 of the vibrationally excited NO2 molecules during collisional quenching was obtained from the IR spectra by modeling the ν3 and ν1+ν3 bands, using the known harmonic frequencies and anharmonicity constants. The average amount of energy lost per collision 〈ΔE〉 was determined from the 〈E〉 versus time data. The results show that there is a dramatic increase in the amount of energy transferred for all bath gases at NO2 energies above 10 000–12 000 cm−1, which is near the origin of the NO2 A2B2/B2B1 states. This threshold in the energy-transfer rate occurs because of strong vibronic coupling between the X2A1 and A2B2/B2B1 electronic states. The increase in vibration-to-vibration (V-V) energy transfer can be understood within the context of the transiti...

Observation of large vibration‐to‐vibration energy transfer collisions (ΔE≳3500 cm−1) in quenching of highly excited NO2 by CO2 and N2O

Time‐resolved Fourier transform infrared emission spectra, recorded after 475 nm excitation of NO2 in a CO2 or N2O bath, show IR emission from collisionally populated vibrational levels of the bath gas. The frequency of the observed bands proves that the emission arises from either the (1,00,1), (0,2l,1), and/or (0,00,2) levels of CO2 or N2O. From the pressure dependence of the emission intensity it was determined that these levels are populated by single collisions with excited NO2. Under typical conditions (1:10 ratio of NO2 to bath gas and 1–2 Torr total pressure) a steady state concentration is reached in our experiments where 0.016±0.006 multiply excited CO2 molecules, or 0.03±0.01 multiply excited N2O molecules were generated per laser excited NO2. A transition dipole coupling model is applied to explain these results, where the resonance conditions for vibration‐to‐vibration energy transfer are relaxed by extensive vibronic and vibrational couplings in highly excited NO2. In this model the energy‐d...

Intramolecular electronic coupling enhanced collisional deactivation of highly vibrationally excited molecules

The collisional deactivation of highly vibrationally excited NO2 and CS2 by a variety of buffer gases has been examined by time‐resolved Fourier transform IR emission spectroscopy. The results show that there is a dramatic increase in the average energy removed per collision for NO2 excited above ∼10 000 cm−1 and for CS2 above ∼26 000 cm−1. These energies correspond to the origins of the lowest excited A 2B2/B 2B1 states of NO2 and the lowest excited R 3A2 state of CS2. Mixing between these excited electronic states with the ground electronic state enhances collisional relaxation by allowing the electronic transition dipole to contribute to collisional energy transfer.

Renner–Teller effect on the highly excited bending levels of ã 1A1 CH2

The effect of Renner–Teller coupling between the a 1A1 and b 1B1 states of CH2 on the rotational structure of the a 1A1 bending vibrational levels has been observed. Renner–Teller coupling causes a decrease in the A rotational constant of the a 1A1 (0,5,0) level, at 6400 cm−1 above the zero‐point level, compared to the value extrapolated from the (0,v2,0) v2=0–4 levels. Excellent agreement is obtained between the experimentally determined A value for (0,5,0) and that predicted by ab initio calculations of Green et al. [J. Chem. Phys. 94, 118 (1991)]. The effect of Renner–Teller coupling on the A rotational constant of bending levels as low as v2=3 has also been detected. The barrier height to linearity in the a 1A1 state was also estimated by fitting the a 1A1 bending level term values to a harmonic plus Gaussian perturbation potential function, where the effects of orbital angular momentum were explicitly included to account for electronic‐rotational coupling in the calculation. The value of 8600±4...

State‐to‐state rotational energy transfer and reaction with ketene of highly vibrationally excited b̃ 1B1 CH2 by time‐resolved Fourier transform emission spectroscopy

Dispersed fluorescence spectra from the CH2 b 1B1→a 1A1 band were recorded with time‐resolution by Fourier transform emission spectroscopy after pulsed excitation of a single rotational level of the b 1B1 (0,160,0) state. Fluorescence observed from the initially excited level and from levels populated by rotational energy changing collisions with the bath gas (ketene) was used to deduce the state‐to‐state rate constants for rotational energy transfer and the state‐resolved rate constants for total collisional removal of b 1B1 CH2. The observed propensity rules for rotational energy transfer—ΔJ=±2, ΔKa=0, and ΔKc=±2—are consistent with a quadrupole–dipole interaction between b 1B1 (0,160,0) CH2 and ketene. The existence of a quadrupole in the intermolecular interaction suggests that the structure of CH2 in the b 1B1 (0,160,0) state, averaged over the time of a collision, must be linear. The state‐to‐state rotational energy transfer rate constants range from approximately equal to the hard sphere gas ...

Strong asymmetry induced ΔKa=3 transitions in the CH2 b̃ 1B1→ã 1A1 spectrum: A study by Fourier transform emission spectroscopy

Dispersed fluorescence spectra of the CH2b 1B1→a 1A1’2116 band were recorded by Fourier transform emission spectroscopy for a series of rotational levels in the b1 B1 (0,160,0) state. Strong ΔKa=3 transitions were observed with their intensity increasing as J increases in the a 1A1 (0,1,0) state. The observed intensities could be well reproduced by a calculation based on a rigid asymmetric rotor Hamiltonian, in which CH2 was assumed to be linear in the b 1B1 state and bent in the a1A1 state. The calculation shows that the intensity of the ΔKa=3 transitions arises from the asymmetry of the a 1A1 state.

Collisional Deactivation of Highly Vibrationally Excited SO2: A Time-Resolved FTIR Emission Spectroscopy Study

Time-resolved Fourier transform IR emission spectroscopy, capable of 10-8 s and 0.1 cm-1 spectral resolution, has been used to study the collisional deactivation of highly vibrationally excited SO2 by bath-gas molecules Ar, N2, O2, CO2 and SF6. The vibrationally excited SO2 were initially prepared with 32,500 cm-1 energy in the X˜1A1 state by the pulsed 308 nm laser excitation followed by internal conversion. The entire collisional deactivation process of the excited SO2 was monitored by time-resolved IR emission spectra through the IR active transitions. The average energy, , of excited SO2 was extracted from the IR emission bands using known vibrational constants and selection rules. is further used to derive the average energy loss per collision, , by each of the bath-gas molecules. The results show that increases from mono- and di-atomic quenchers to more complex polyatomic molecules, as V-V energy transfer contributes to V-T/R. For all bath molecules, increases with and displays a marked increase at ≈ 20,000 cm-1. The observed threshold behavior most likely arises from intramolecular vibronic coupling within SO2 and implies the importance of long range interaction in intermolecular energy transfer.