Jet-Cooled Phosphorescence Excitation Spectrum of the T1(n,π*) ← S0 Transition of 4H-Pyran-4-one.

The 4H-pyran-4-one (4PN) molecule is a cyclic conjugated enone with spectroscopically accessible singlet and triplet (n,π*)excited states. Vibronic spectra of 4PN provide a stringent test of electronic-structure calculations, through comparison of predicted vs measured vibrational frequencies in the excited state. We report here the T1(n,π*) ← S0 phosphorescence excitation spectrum of 4PN, recorded under the cooling conditions of a supersonic free-jet expansion. The jet cooling has eliminated congestion appearing in previous room-temperature measurements of the T1 ← S0 band system and has enabled us to determine precise fundamental frequencies for seven vibrational modes of the molecule in its T1(n,π*) state. We have also analyzed the rotational contour of the 000 band, obtaining experimental values for spin-spin and spin-rotation constants of the T1(n,π*) state. We used the experimental results to test predictions from two commonly used computational methods, equation-of-motion excitation energies with dynamical correlation incorporated at the level of coupled cluster singles doubles (EOM-EE-CCSD) and time-dependent density functional theory (TDDFT). We find that each method predicts harmonic frequencies within a few percent of observed fundamentals, for in-plane vibrational modes. However, for out-of-plane modes, each method has specific liabilities that result in frequency errors on the order of 20-30%. The calculations have helped to identify a perturbation from the T2(π,π*) state that leads to unexpected features observed in the T1(n,π*) ← S0 origin band rotational contour.

Dual chirped-pulse electro-optical frequency comb method for simultaneous molecular spectroscopy and dynamics studies: formic acid in the terahertz region.

An electro-optic dual-comb system based on chirped-pulse waveforms is used to simultaneously acquire temporally magnified rapid passage signals and normal spectral line shapes from the back-transformation to the time domain. Multi-heterodyne terahertz (THz) wave generation and detection is performed with the difference frequency mixing of two free-running lasers. The method is used to obtain THz spectra of formic acid in the 10 cm-1 to 20 cm-1 (300 GHz-600 GHz) region over a range of pressures. The method is widely applicable across other spectral regions for investigations of the transient dynamics and spectroscopy of molecular systems.

Difference-frequency chirped-pulse dual-comb generation in the THz region: Temporal magnification of the quantum dynamics of water vapor lines by >60 000.

A new difference-frequency method based on electro-optic phase modulators (EOMs) and two free-running lasers is reported to perform chirped-pulse dual-comb spectroscopy in the THz region. A variation of a near-IR interleaving scheme we recently reported has been developed to interleave the EOMs' orders and sidebands and to map THz comb teeth into the radio-frequency region below 1 MHz. The down-converted comb teeth are shown to have transform limited widths of 1 Hz over a 1 s time scale. The dual chirp-pulsed scheme is used to measure the complex line shapes of two water vapor lines below 600 GHz and to temporally magnify the effects of rapid passage by more than 60 000. For the 11,0 ← 10,1 transition in H2O, a pressure dependent phase perturbation is observed in the rapid passage response over the magnified time scale in contrast to a uniform line shape transformation observed for the 21,1 ← 20,2 transition of D2O. The possible origins for this anomalous behavior are modeled and discussed. The method is applicable to any region where difference or sum frequency waves can be generated.

Interleaved electro-optic dual comb generation to expand bandwidth and scan rate for molecular spectroscopy and dynamics studies near 1.6 µm.

A chirped-pulse interleaving method is reported for generation of dual optical frequency combs based on electro-optic phase modulators (EOM) in a free-running all-fiber based system. Methods are discussed to easily modify the linear scan rate and comb resolution by more than three orders of magnitude and to significantly increase the spectral bandwidth coverage. The agility of the technique is shown to both capture complex line shapes and to magnify rapid passage effects in spectroscopic and molecular dynamics studies of CO2. These methods are well-suited for applications in the areas of remote sensing of greenhouse gas emissions, molecular reaction dynamics, and sub-Doppler studies across the wide spectral regions accessible to EOMs.

Rotationally resolved UV spectroscopy of the rotamers of indole-4-carboxylic acid: Evidence for charge transfer quenching.

Rotationally resolved electronic spectra of two conformational isomers of jet-cooled indole-4-carboxylic acid (I4CA) and the deuterated forms of the acid (-COOD) and amide (-ND) groups have been obtained using a UV laser/molecular beam spectrometer. The in-plane orientation of the acid group defines the two lowest energy rotamers of I4CA. The S1 ← S0 origin bands of the two rotamers and four isotopologues have been fit to asymmetric rotor Hamiltonians in both electronic states. From the best-fit parameters, the positions of the H-atoms in the principal axis frames of each conformer have been determined and serve to unambiguously identify the syn forms (i.e., COH⋯O) of the cis and trans rotamers. The experimental S0 and S1 inertial parameters, hydrogen atom positions, and transition dipole moment (TDM) orientations are compared with the results of theoretical calculations. The TDM orientation indicates that the S1 state is the 1La state in contrast to most substituted indoles. The molecular orbital properties and natural charges are investigated to better understand the 1La/1Lb state reversal and the extent of photoinduced intramolecular charge transfer that impacts the rotamer-dependent fluorescence lifetimes.

Multi-frequency differential absorption LIDAR system for remote sensing of CO2 and H2O near 1.6 µm.

The specifications and performance of a ground-based differential absorption LIDAR (light detection and ranging) system (DIAL) using an optical parametric oscillator (OPO) are presented. The OPO is injection-seeded with the output of a confocal filter cavity at frequencies generated by an electro-optic phase modulator (EOM) from a fixed-frequency external cavity diode laser (ECDL). The number of seed frequencies, frequency spacings, and duration is controlled with an arbitrary waveform generator (AWG) driving the EOM. Range resolved data are acquired using both photon current and photon counts from a hybrid detection system. The DIAL measurements are performed using a repeating sequence of 10 frequencies spanning a range of 37.5 GHz near 1602.2 nm to sequentially sample CO2 and H2O at 10 Hz. Dry air mixing ratios of CO2 and H2O with a resolution of 250 m and an averaging time of 10 min resulted in uncertainties as low as 6 µmol/mol (ppm) and 0.44 g/kg, respectively. Simultaneous measurements using an integrated path differential absorption (IPDA) LIDAR system and in situ point sensor calibrated to WMO (World Meteorological Organization) gas standards are conducted over two 10 hr nighttime periods to support traceability of the DIAL results. The column averaged DIAL mixing ratios agree with the IPDA LIDAR results to within the measured uncertainties for much of two measurement periods. Some of the discrepancies with the in situ point sensor results are revealed through trends observed in the gradients of the range resolved DIAL data.

Ground-based, integrated path differential absorption LIDAR measurement of CO2, CH4, and H2O near 1.6 µm.

A ground-based, integrated path, differential absorption light detection and ranging (IPDA LIDAR) system is described and characterized for a series of nighttime studies of CO2, CH4, and H2O. The transmitter is based on an actively stabilized, continuous-wave, single-frequency external-cavity diode laser (ECDL) operating from 1.60 to 1.65 µm. The fixed frequency output of the ECDL is microwave sideband tuned using an electro-optical phase modulator driven by an arbitrary waveform generator and filtered using a confocal cavity to generate a sequence of 123 frequencies separated by 300 MHz. The scan sequence of single sideband frequencies of 600 ns duration covers a 37 GHz region at a spectral scan rate of 10 kHz (100 µs per scan). Simultaneously, an eye-safe backscatter LIDAR system at 1.064 µm is used to monitor the atmospheric boundary layer. IPDA LIDAR measurements of the CO2 and CH4 dry air mixing ratios are presented in comparison with those from a commercial cavity ring-down (CRD) instrument. Differences between the IPDA LIDAR and CRD concentrations in several cases appear to be well correlated with the atmospheric aerosol structure from the backscatter LIDAR measurements. IPDA LIDAR dry air mixing ratios of CO2 and CH4 are determined with fit uncertainties of 2.8 µmol/mol (ppm) for CO2 and 22 nmol/mol (ppb) for CH4 over 30 s measurement periods. For longer averaging times (up to 1200 s), improvements in these detection limits by up to 3-fold are estimated from Allan variance analyses. Two sources of systematic error are identified and methods to remove them are discussed, including speckle interference from wavelength decorrelation and the seed power dependence of amplified spontaneous emission. Accuracies in the dry air retrievals of CO2 and CH4 in a 30 s measurement period are estimated at 4 µmol/mol (1% of ambient levels) and 50 nmol/mol (3%), respectively.

Coherent cavity-enhanced dual-comb spectroscopy.

Dual-comb spectroscopy allows for the rapid, multiplexed acquisition of high-resolution spectra without the need for moving parts or low-resolution dispersive optics. This method of broadband spectroscopy is most often accomplished via tight phase locking of two mode-locked lasers or via sophisticated signal processing algorithms, and therefore, long integration times of phase coherent signals are difficult to achieve. Here we demonstrate an alternative approach to dual-comb spectroscopy using two phase modulator combs originating from a single continuous-wave laser capable of > 2 hours of coherent real-time averaging. The dual combs were generated by driving the phase modulators with step-recovery diodes where each comb consisted of > 250 teeth with 203 MHz spacing and spanned > 50 GHz region in the near-infrared. The step-recovery diodes are passive devices that provide low-phase-noise harmonics for efficient coupling into an enhancement cavity at picowatt optical powers. With this approach, we demonstrate the sensitivity to simultaneously monitor ambient levels of CO2, CO, HDO, and H2O in a single spectral region at a maximum acquisition rate of 150 kHz. Robust, compact, low-cost and widely tunable dual-comb systems could enable a network of distributed multiplexed optical sensors.

Tunable resolution terahertz dual frequency comb spectrometer.

Terahertz dual frequency comb spectroscopy (THz-DFCS) yields high spectral resolution without compromising bandwidth. Nonetheless, the resolution of THz-DFCS is usually limited by the laser repetition rate, which is typically between 80 MHz and 1 GHz. In this paper, we demonstrate a new method to achieve sub-repetition rate resolution in THz-DFCS by adaptively modifying the effective laser repetition rate using integrated Mach-Zehnder electro-optic modulators (MZ-EOMs). Our results demonstrate that it is possible to improve the 100 MHz resolution of a terahertz frequency comb by at least 20x (down to 5 MHz) across the terahertz spectrum without compromising the average output power, and to a large extent, its bandwidth. Our approach can augment a wide range of existing THz-DFCS systems to provide a significant and easily adaptable resolution improvement.

Vibronic coupling in asymmetric bichromophores: experimental investigation of diphenylmethane-d5.

Vibrationally and rotationally resolved electronic spectra of diphenylmethane-d5 (DPM-d5) are reported in the isolated-molecule environment of a supersonic expansion. While small, the asymmetry induced by deuteration of one of the aromatic rings is sufficient to cause several important effects that change the principle mechanism of vibronic coupling between the close-lying S1 and S2 states, and spectroscopic signatures such coupling produces. The splitting between S1 and S2 origins is 186 cm(-1), about 50% greater than its value in DPM-d0 (123 cm(-1)), and an amount sufficient to bring the S2 zero-point level into near-resonance with the v = 1 level in the S1 state of a low-frequency phenyl flapping mode, ν(R) = 191 cm(-1). Dispersed fluorescence spectra bear clear evidence that Δv(R) = 1 Herzberg-Teller coupling dominates the near-resonant internal mixing between the S1 and S2 manifolds. The fluorescence into each pair of Franck-Condon active ring modes shows an asymmetry that suggests incorrectly that the S1 and S2 states may be electronically localized. From rotationally resolved studies, the S0 and S1 states have been well-fit to asymmetric rotor Hamiltonians while the S2 state is perturbed and not fit. The transition dipole moment (TDM) orientation of the S1 state is nearly perpendicular to the C2 symmetry axes with 66(2)%:3(1)%:34(2)% a:b:c hybrid-type character while that of the S2 origin contains 50(10)% a:c-type (S1) and 50(10)% b-type (S2) character. A model is put forward that explains qualitatively the TDM compositions and dispersed emission patterns without the need to invoke electronic localization. The experimental data discussed here serve as a foundation for a multi-mode vibronic coupling model capable of being applied to asymmetric bichromophores, as presented in the work of B. Nebgen and L. V. Slipchenko ["Vibronic coupling in asymmetric bichromophores: Theory and application to diphenylmethane-d5," J. Chem. Phys. (submitted)].

Segmented chirped-pulse Fourier transform submillimeter spectroscopy for broadband gas analysis.

Chirped-pulse Fourier transform spectroscopy has recently been extended to millimeter wave spectroscopy as a technique for the characterization of room-temperature gas samples. Here we present a variation of this technique that significantly reduces the technical requirements on high-speed digital electronics and the data throughput, with no reduction in the broadband spectral coverage and no increase in the time required to reach a given sensitivity level. This method takes advantage of the frequency agility of arbitrary waveform generators by utilizing a series of low-bandwidth chirped excitation pulses paired in time with a series of offset single frequency local oscillators, which are used to detect the molecular free induction decay signals in a heterodyne receiver. A demonstration of this technique is presented in which a 67 GHz bandwidth spectrum of methanol (spanning from 792 to 859 GHz) is acquired in 58 µs.

Excitonic splitting and vibronic coupling in 1,2-diphenoxyethane: conformation-specific effects in the weak coupling limit.

Vibrationally and rotationally resolved electronic spectra of 1,2-diphenoxyethane (C6H5-O-CH2-CH2-O-C6H5, DPOE) are reported for the isolated molecule under jet-cooled conditions. The spectra demonstrate that the two excited surfaces are within a few cm(-1) of one another over significant regions of the torsional potential energy surfaces that modulate the position and orientation of the two aromatic rings with respect to one another. Two-color resonant two-photon ionization (2C-R2PI) and laser-induced fluorescence excitation spectra were recorded in the near-ultraviolet in the region of the close-lying S0-S1 and S0-S2 states (36,400-36,750 cm(-1)). In previous work, double resonance spectroscopy in the ultraviolet and alkyl CH stretch regions of the infrared was used to identify and assign transitions to two conformational isomers differing primarily in the central C-C dihedral angle, a tgt conformation with C2 symmetry and a ttt conformation with C2h symmetry [E. G. Buchanan, E. L. Sibert, and T. S. Zwier, J. Phys. Chem. A 117, 2800 (2013)]. Comparison of 2C-R2PI spectra recorded in the m∕z 214 (all (12)C) and m∕z 215 (one (13)C) mass channels demonstrate the close proximity of the S1 and S2 excited states for both conformations, with an upper bound of 4 cm(-1) between them. High resolution spectra of the origin band of the tgt conformer reveal it to consist of two transitions at 36,422.91 and 36,423.93 cm(-1), with transition dipole moments perpendicular to one another. These are assigned to the S0-S1 and S0-S2 origin transitions with excited states of A and B symmetry, respectively, and an excitonic splitting of only 1.02 cm(-1). The excited state rotational constants and transition dipole coupling model directions prove that the electronic excitation is delocalized over the two rings. The ttt conformer has only one dipole-allowed electronic transition (Ag→Bu) giving rise to a pure b-type band at 36,508.77 cm(-1). Here, the asymmetry induced by a single (13)C atom in one of the rings is sufficient to localize the electronic excitation in one or the other ring. Dispersed fluorescence (DFL) spectra are used to provide assignments for all vibronic structure in the first 200 cm(-1)of both conformers. In the tgt conformer, both "a" and "b" symmetry fundamentals are observed, consistent with extensive vibronic coupling between the two dipole-allowed, nearly degenerate excited states. In the ttt conformer, the lowest frequency vibronic transition located 46 cm(-1) above the Bu origin is assigned to a bu fundamental (labeled R[overline]) built off the dipole-forbidden Ag state origin. The DFL spectrum of the Ag(R[overline](1)) level contains strong transitions to v(")(R[overline]) = 0, 1, and 2, seemingly at odds with vibronic coupling models. Studies of the DFL spectrum of this band as a function of distance from the nozzle reveal that much of the intensity in v(") = 1 arises from collisions of DPOE while in the excited state Ag(vb' = 1) level with He, producing Bu(R[overline] = 1) levels with large collision cross section. The remaining intensity in the fundamental at large x∕D is ascribed to emission from the (13)C isotopomer, for which this emission is dipole-allowed.

Chirped-pulse terahertz spectroscopy for broadband trace gas sensing.

We report the first demonstration of a broadband trace gas sensor based on chirp-pulse terahertz spectroscopy. The advent of newly developed solid state sources and sensitive heterodyne detectors for the terahertz frequency range have made it possible to generate and detect precise arbitrary waveforms at THz frequencies with ultra-low phase noise. In order to maximize sensitivity, the sample gas is first polarized using sub-µs chirped THz pulses and the free inductive decays (FIDs) are then detected using a heterodyne receiver. This approach allows for a rapid broadband multi-component sensing with low parts in 10(9) (ppb) sensitivities and spectral frequency accuracy of <20 kHz in real-time. Such a system can be configured into a portable, easy to use, and relatively inexpensive sensing platform.

Rotationally resolved C2 symmetric conformers of bis-(4-hydroxyphenyl)methane: prototypical examples of excitonic coupling in the S1 and S2 electronic states.

Rotationally resolved microwave and ultraviolet spectra of jet-cooled bis-(4-hydroxyphenyl)methane (b4HPM) have been obtained using Fourier-transform microwave and UV laser/molecular beam spectrometers. A recent vibronic level study of b4HPM [Rodrigo, C. P.; Müller, C. W.; Pillsbury, N. R.; James, W. H., III; Plusquellic, D. F.; Zwier, T. S. J. Chem. Phys. 2011, 134, 164312] has assigned two conformers distinguished by the orientation of the in-plane OH groups and has identified two excitonic origins in each conformer. In the present study, the rotationally resolved bands of all four states have been well-fit to asymmetric rotor Hamiltonians. For the lower exciton (S(1)) levels, the transition dipole moment (TDM) orientations are perpendicular to the C(2) symmetry axes and consist of 41(2):59(2) and 34(2):66(2)% a:c hybrid-type character. The S(1) levels are therefore delocalized states of B symmetry and represent the antisymmetric combinations of the zero-order locally excited states of the p-cresol-like chromophores. The TDM polarizations of bands located at ≈132 cm(-1) above the S(1) origins are exclusively b-type and identify them as the upper exciton S(2) origin levels of A symmetry. The TDM orientations and the relative band strengths from the vibronic study have been analyzed within a dipole-dipole coupling model in terms of the localized TDM orientations, µ(loc), on the two chromophores. The out-of-the-ring plane angles of µ(loc) are both near 20° and are similar to results for diphenylmethane [Stearns, J. A.; Pillsbury, N. R.; Douglass, K. O.; Müller, C. W.; Zwier, T. S.; Plusquellic, D. F. J. Chem. Phys. 2008, 129, 224305]. The in-plane angles are, however, rotated by 14 and 18° relative to DPM and, in part, explain the smaller than expected exciton splittings of these two conformers.

On the excited state dynamics of vibronic transitions. High-resolution electronic spectra of acenaphthene and its argon van der Waals complex in the gas phase.

Rotationally resolved fluorescence excitation spectroscopy has been used to study the dynamics, electronic distribution, and the relative orientation of the transition moment vector in several vibronic transitions of acenaphthene (ACN) and in its Ar van der Waals (vdW) complex. The 0(0)(0) band of the S(1) ← S(0) transition of ACN exhibits a transition moment orientation parallel to its a-inertial axis. However, some of the vibronic bands exhibit a transition moment orientation parallel to the b-inertial axis, suggesting a Herzberg-Teller coupling with the S(2) state. Additionally, some other vibronic bands exhibit anomalous intensity patterns in several of their rotational transitions. A Fermi resonance involving two near degenerate vibrations has been proposed to explain this behavior. The high-resolution electronic spectrum of the ACN-Ar vdW complex has also been obtained and fully analyzed. The results indicate that the weakly attached argon atom is located on top of the plane of the bare molecule at ~3.48 Šaway from its center of mass in the S(0) electronic state.

Conformer-specific vibronic spectroscopy and vibronic coupling in a flexible bichromophore: bis-(4-hydroxyphenyl)methane.

The vibronic spectroscopy of jet-cooled bis-(4-hydroxyphenyl)methane has been explored using fluorescence excitation, dispersed fluorescence (DFL), UV-UV hole-burning, UV depletion, and fluorescence-dip infrared spectroscopies. Calculations predict the presence of three nearly isoenergetic conformers that differ in the orientations of the two OH groups in the para positions on the two aromatic rings (labeled uu, dd, and ud). In practice, two conformers (labeled A and B) are observed, with S(0)-S(1) origins at 35,184 and 35,209 cm(-1), respectively. The two conformers have nearly identical vibronic spectra and hydride stretch infrared spectra. The low-frequency vibronic structure is assigned to bands involving the phenyl torsions (T and T), ring-flapping (R and R), and butterfly (ß) modes. Symmetry arguments lead to a tentative assignment of the two conformers as the C(2) symmetric uu and dd conformers. The S(0)-S(2) origins are assigned to bands located 132 cm(-1) above the S(0)-S(1) origins of both conformers. DFL spectra from the S(2) origin of the two conformers display extensive evidence for vibronic coupling between the two close-lying electronic states. Near-resonant coupling from the S(2) origin occurs dominantly to S(1) R(1) and S(1) R(1)ß(1) levels, which are located -15 and +31 cm(-1) from it. Unusual vibronic activity in the ring-breathing (ν(1)) and ring-deformation (ν(6a)) modes is also attributed to vibronic coupling involving these Franck-Condon active modes. A multimode vibronic coupling model is developed based on earlier theoretical descriptions of molecular dimers [Fulton and Gouterman, J. Chem. Phys. 35, 1059 (1961)] and applied here to flexible bichromophores. The model is able to account for the ring-mode activity under conditions in which the S(2) origin is strongly mixed (60%/40%) with S(1) 6a(1) and 1(1) levels. The direct extension of this model to the T/T and R/R inter-ring mode pairs is only partially successful and required some modification to lower the efficiency of the S(1)/S(2) mixing compared to the ring modes.

State-resolved THz spectroscopy and dynamics of crystalline peptide-water systems.

Vibrationally state-resolved THz spectra are obtained at cryogenic temperatures for three crystalline peptide-water systems that represent different structural motifs. The systems include two types of secondary structures and a hydrophobic peptide nanopore structure. Almost all of these systems are shown to undergo exchange with water at room temperature that alters the hydrogen bonding network in ways easily detectable in the THz region at cryogenic temperatures. Stark differences are observed in the spectra of model alpha-helical and beta-sheet structures upon water removal at hydrophilic binding sites. However, within the confined pore of a hydrophobic nanotube, water in the form of helical wires has a subtle but significant impact on the phonon modes of the tube. The THz spectra are shown to easily distinguish between the different hydration states of the system that have been independently characterized by mass change measurements. Spectral comparisons with quantum chemical predictions of fully relaxed crystal structures confirm the hydration states and give detailed information about the free energies associated with dehydration. The vibrational free energies are shown to make significant contributions to the overall energy balance of the dehydration processes.

Conformational effects on excitonic interactions in a prototypical H-bonded bichromophore: bis(2-hydroxyphenyl)methane.

Laser-induced fluorescence, single-vibronic level fluorescence (SVLF), UV hole burning, and fluorescence dip infrared (FDIR) spectroscopy have been carried out on bis-(2-hydroxyphenyl)methane in order to characterize the ground-state and first excited-state vibronic spectroscopy of this model flexible bichromophore. These studies identified the presence of two conformational isomers. The FDIR spectra in the OH-stretch region determine that conformer A is an OH...O H-bonded conformer, while conformer B is a doubly OH...pi H-bonded conformer with C(2) symmetry. High-resolution ultraviolet spectra ( approximately 50 MHz resolution) of a series of vibronic bands of both conformers confirm and refine these assignments. The transition dipole moment (TDM) direction in conformer A is consistent with electronic excitation that is primarily localized on the donor phenol ring. A tentative assignment of the S(2) origin is made to a set of transitions approximately 400 cm(-1) above S(1). In conformer B, the TDM direction firmly establishes C(2) symmetry for the conformer in its S(1) state and establishes the electronic excitation as delocalized over the two rings, as the lower member of an excitonic pair. The S(2) state has not been clearly identified in the spectrum. Based on CIS calculations, the S(2) state is postulated to be several times weaker than S(1), making it difficult to identify, especially in the midst of overlap from vibronic bands due to conformer A. SVLF spectra show highly unusual vibronic intensity patterns, particularly in conformer B, which cannot be understood by simple harmonic Franck-Condon models, even in the presence of Duschinsky mixing. We postulate that these model flexible bichromophores have TDMs that are extraordinarily sensitive to the distance and orientation of the two aromatic rings, highlighting the need to map out the TDM surface and its dependence on the (up to) five torsional and bending coordinates in order to understand the observations.

State-specific studies of internal mixing in a prototypical flexible bichromophore: Diphenylmethane.

Laser-induced fluorescence, resonant two-photon ionization, UV-UV hole burning, UV depletion, and single vibronic level fluorescence (SVLF) spectra of jet-cooled diphenylmethane (DPM) have been recorded over the 37 300-38 400 cm(-1) region that encompasses the S(1)<--S(0) and S(2)<--S(0) transitions. All transitions in the laser-induced fluorescence excitation spectrum are due to a single conformational isomer of DPM with C(2) symmetry. The S(1)<--S(0) origin transition occurs at 37 322 cm(-1), supporting a short progression in the symmetric torsion T with spacing of 28 cm(-1). The S(2)<--S(0) origin transition occurs 123 cm(-1) above the S(1) origin and possesses very weak torsional structure, observable only under saturating laser power conditions. A combination of SVLF spectroscopy and hot band studies is used to assign the frequencies of the symmetric torsion (T), antisymmetric torsion (T), and butterfly (beta) vibrations in the S(0), S(1), and S(2) states. The emission from the S(2) origin is composed of two components, a set of sharp transitions ascribable to the S(2) state and a dense "clump" of transitions ending in ground-state levels 81, 88, and 93 cm(-1) above the S(0) zero-point level ascribable to S(1)(v) emission. Assignment of the transitions in the clump leads to the conclusion that the single vibronic level responsible for the emission has mixed S(2)/S(1) character. The mixing involves several torsional vibronic levels in the S(1) manifold close in energy to the S(2) origin, with the correct symmetry to couple the two states. These levels involve significant torsional excitation. The close energetic proximity of these levels leads to a breakdown of typical vibronic coupling selection rules.