High resolution UV spectroscopy of aromatic molecules


Abstract

In an ongoing research project we aim at a measurement of the electronic spectra of large organic molecules with the highest possible spectral resolution. The combination of an intra-cavity frequency doubled cw ring dye laser with a carefully designed molecular beam machine and a sensitive laser induced fluorescence detection set-up has yielded a molecular beam spectrometer with a spectral resolution of 1 part in 10^8 over the whole near-UV range of the spectrum. Rotationally resolved electronic spectra of increasingly challenging polyatomic molecules as well as of their van der Waals and hydrogen bonded complexes are measured in this apparatus, and their geometrical structures are deduced. In the near future double-resonance techniques, using microwave or infrared preselection of one ro-vibrational level which is then probed by the tunable UV laser, will be employed to expand the applicability of this spectrometer to even larger molecules.

Introduction

The combination of a supersonic molecular beam expansion and a narrow band UV laser is a powerful tool in experimental molecular spectroscopy. It can provide detailed information about the dynamics and structure of molecules and molecular complexes in both their ground and electronically excited states. Expanding volatilized organic molecules seeded in a carrier gas produces a cooling of the vibrational and rotational degrees of freedom. The advantage for high resolution spectroscopy is two-fold. On the one hand, only the lowest rotational and vibrational levels in the electronic ground state are populated, leading to less congested excitation spectra. On the other hand, the low internal temperatures permit the stabilization of structural variants (tautomers or conformers) and the stabilization of molecular clusters (van der Waals and hydrogen bonded complexes).

Analysis of rotationally resolved laser induced fluorescence (LIF) spectra provides the molecular constants in both the ground and the electronically excited state. These constants are directly related to the geometrical structures in both states, giving access to information about intramolecular bond lengths, and in the case of a molecular complexes, intermolecular bond lengths and their changes upon excitation.

Unfortunately, the number of molecular constants is too small for the determination of the complete molecular structure. Consider, for example, a molecule with N atoms that can be described with an asymmetric rigid rotor Hamiltonian. There are only three rotational constants A, B and C available, while there are 3N-3 unknown parameters. Recording the rotationally resolved LIF spectrum of an isotopically substituted molecule can give extra information to determine the position of the substituted atom via Kraitchman's equations. Obviously, in the case of a molecule containing for example 30 atoms, determination of all atomic positions would be a very tedious process. This method is used if only a particular part of the molecule is interesting, such as the NH2 group in 1-aminonaphthalene.

If the molecule consists of parts with a well-known structure, the rotational constants of the entire molecule contain enough information to determine its structure. An example is triphenylamine (TPA), a molecule which consists of a nitrogen atom with three phenyl groups attached to it. Since the structure of each phenyl group is known, there are only a few unknown parameters left, which are related to the relative orientation of the phenyl groups. Therefore, it is possible to determine the structure of the entire molecule. In all other cases the molecular constants have to be compared with other (related) molecules or with ab initio calculations to provide information about the structure. This last comparison is very important; it is a sensitive check of the methods of calculation that are frequently used for providing a large amount of information about molecular properties.

In addition to the molecular constants, one can determine the orientation of the electronic transition moment vector in the molecular frame from the high resolution spectrum. This vector provides information about the direction of the electronic charge migration or displacement that occurs during the transition. It is therefore related to the probability distribution functions in the involved electronic states. Furthermore, from a deconvolution of the rotational line shape, the natural linewidth of the molecular transition can be obtained which gives the lifetime of the excited state.

High resolution UV spectroscopy also can provide information about interactions between electonic states. As an example, fluorescence excitation spectra can be perturbed by a coupling with `dark' states (ISC, intersystem crossing). The excitation spectrum of pyrazine contains many more lines than expected, owing to a coupling between single rovibronic levels of the S1 state with many quasi-isoenergetic rovibronic levels of the lowest triplet state (T1). Similar perturbations have been observed in the spectra of pyrimidine, sym-triazine and acetylene.

Another interesting interaction is caused by the coupling of an internal hindered rotation with the overall rotation of the molecule. Full analysis of the spectra can provide values for the barrier heights in the ground state and the excited state. The extent of complexity of the spectra depends strongly on the barrier heights, the direction of the internal rotation axis with respect to the overall inertial axis, and the (optical) selection rules (type of transition). In phenol, every rotational line is split into two barely resolved components due to the torsion of the hydroxyl group around the C--O bond. More complex is methylindole: internal rotation of the methyl group leads to a spectrum which consists of two bands (A and E lines), in which the E lines are further split by a Ka-dependent interaction.

A number of high resolution UV experiments have been performed covering most of the aforementioned topics. The spectrometer consists of a molecular beam apparatus and an intracavity frequency-doubled continuous-wave (cw) ring dye laser (265--340 nm), and has a spectral resolution of 1 part in 10^8. The studied molecular systems differ largely in size: from a single molecule containing 12 atoms (phenol) to a cluster containing 41 atoms (the van der Waals complex of 1-cyanonaphthalene and triethylamine).

Experimental set-up

Rotationally resolved fluorescence excitation spectra are obtained using a narrow bandwidth UV laser system and a molecular beam apparatus. The sample is heated in a quartz oven to bring it into the gas phase, seeded in 0.2-1.0 bar argon, and expanded through a nozzle with a diameter of 0.15 mm. The nozzle is kept at a slightly higher temperature to prevent condensation of the sample in the orifice. The molecular beam is skimmed twice in a differential pumping system and is crossed perpendicularly with a UV laser beam at about 30 cm from the nozzle.

UV radiation with a bandwidth of 3 MHz is generated by intracavity frequency doubling in a single frequency ring dye laser. Typically 0.1--5 mW of tunable radiation can be obtained. For relative frequency calibration a temperature stabilized Fabry-Perot interferometer is used with a free spectral range of 75 MHz. For absolute frequency calibration, the iodine absorption spectrum is recorded simultaneously. The total undispersed fluorescence is imaged on a photomultiplier connected to a photon counting system interfaced with a computer.


Experiments

Triphenylamine and its Van der Waals Complexes

Both vibrationally and rotationally resolved spectra of the S1 <-- S0 transition in jet-cooled triphenylamine (TPA) around 340-320 nm are reported. Medium resolution spectra (0.5-1.0 cm-1 resolution) are recorded using (1+1)-Resonance Enhanced Multi Photon Ionization (REMPI) with mass selective Time-Of-Flight (TOF) detection in a pulsed molecular beam apparatus. The origin of the S1 <-- S0 transition is at 29520.7 cm-1, higher than halfway to the ionization potential (IP) found at 6.89 eV. A vibrational progression in the symmetric torsion mode (114 cm-1) as well as in the symmetric C--N stretching mode (280 cm-1) is observed in the electronic spectra. The spectrum of the most abundant isomer of the TPA--Ar (TPA--Kr) complexes is blue-shifted by 211 cm-1 (216 cm-1) with respect to the spectrum of the free TPA molecule. High resolution spectra are recorded using Laser Induced Fluorescence (LIF) in a cw molecular beam apparatus. Individual rotational transitions are resolved and the spectrum shows unambiguously that TPA is a symmetric top molecule. The spectrum of the blue-shifted TPA--Ar isomer is the spectrum of a symmetric top molecule as well, and therefore the Ar atom has to be located on the C3 symmetry axis, either on top of or underneath the umbrella formed by the phenyl rings. It appears that when Ar or Kr forms a complex with TPA, the first Ar, Kr, atom goes preferentially in a position on the C3 symmetry axis of TPA, a position which causes an abnormal blue-shift of the spectrum. With the first rare gas atom located in this special position, the second rare gas atom is forced into a `normal' position, i.e. above one of the phenyl-rings, causing a normal red-shift with respect to the TPA--Ar complex. (ref)

Microwave-ultraviolet double resonance spectroscopy on 1-cyanonaphthalene

The rotationally resolved fluorescence excitation spectrum of the 0-0 band in the S1 <-- S0 transition of 1-cyanonaphthalene (CNN), at 318 nm, has been recorded using laser induced fluorescence in a molecular beam apparatus. This band exhibits pure a-type character and consists of about 600 lines at a rotational temperature of 2.5 K, each with a linewidth of 17 MHz. A microwave-ultraviolet double resonance experiment on the 0-0 band of CNN has been performed to verify the rotational assignments of the fluorescence excitation spectrum and to obtain more accurate rotational constants in both the ground and electronically excited states. The band origin is at 31411.114 ±0.003 cm-1 and the rotational constants are (in MHz) A''=1478.65(2), B''=956.75(1), C''=580.989(7), A'-A''=-21.363(9), B'-B''=-13.305(5), and C'-C''=-8.167(2).(ref)
Spectra 
Cyanonaphthalene (upper: MW-UV double resonance)

1-cyanonaphthalene/triethylamine van der Waals complex

The rotationally resolved fluorescence excitation spectrum of the 0-0 band in the S1 <-- S0 transition, at 318 nm, of the 1-cyanonaphthalene/triethylamine van der Waals complex has been recorded using laser induced fluorescence in a molecular beam apparatus. This spectrum could be fitted to a pure a-type band. From the rotational constants a T-shaped geometry could be deduced.(ref)

1-aminonaphthalene

The high resolution fluorescence excitation spectrum of the origin band of the S1 <-- S0 transition of 1-aminonaphthalene (1AN) has been recorded. It was found that this band is predominantly b-axis polarized, in contrast with other (previously measured) 1-substituted naphthalenes, which are a-axis polarized. Thirteen vibronic bands of 1AN were also examined at high resolution. The rotational constants, the inertial defects, and the band polarizations vary significantly from band to band. Similar experiments have been performed on eight deuterated isotopomers. A comparison of the results obtained for these isotopomers with those of the corresponding bands in protonated 1AN makes possible the determination of the center-of-mass coordinates of the amino hydrogen atoms. In the zero point vibrational level (ZPL) of the S0 state, the out-of-plane positions of the amino hydrogens are inequivalent; the `inside' hydrogen is located 0.49(8) Angstrom out-of-plane, the `outside' hydrogen only 0.24(17) Angstrom. In the ZPL of the S1 state, 1AN is quasi-planar.(ref)

2H-tautomer of benzotriazole

The rotationally resolved excitation spectrum of the 0-0 band of the S1 <-- S0 transition in 2H-benzotriazole, at 286.4 nm, is obtained by using laser induced fluorescence spectroscopy in a molecular beam. From this pure b-type spectrum, the rotational constants in the ground state and the electronically excited state are determined. The rotational lines are strongly broadened due to the short lifetime which is determined to be around 1.1 ns.(ref)

indole, indazole and benzimidazole

Rotationally resolved laser induced fluorescence excitation spectra of the S1 (1Lb) <-- S0 origin bands of indole, indazole, and benzimidazole have been measured. From these spectra, the rotational constants in both electronic states have been determined. The Indole   Axis
 reorientation spectra of all three molecules exhibit `anomalous' rotational line intensities. These intensity perturbations are a result of the reorientation, upon electronic excitation, of the inertial axes of the molecule. Intensity analysis of the rotational lines yielded information about the inertial axis reorientation, and the direction of the transition moment vector for each molecule.(ref)

Phenol and the hydrogen bonded phenol-water cluster

'Water rotates!' The S1 <-- S0 0-0 transitions of phenol and the hydrogen bonded phenol-water cluster have been studied by high resolution fluorescence excitation spectroscopy. All lines in the monomer spectrum are split by 56±4 MHz due to the internal rotation of the --OH group about the a-axis. The barrier for this internal motion is determined in the ground and excited states; V_2''=1215 cm-1, and V_2'=4710 cm-1. The rotational constants for the monomer in the ground state are in agreement with those reported in microwave studies. The excited state rotational constants were found to be A'=5313.7 MHz, B'=2620.5 MHz, and C'=1756.08 MHz. The region of the redshifted 0-0 transition of phenol-water shows two distinct bands which are 0.85 cm-1 apart. Their splitting arises from a torsional motion which interchanges the two equivalent H-atoms in the H2O moiety of the cluster. This assignment was confirmed by spin statistical considerations. Both bands could be fit to rigid rotor Hamiltonians. Due to the interaction between the overall rotation of the entire cluster and the internal rotation, both bands have different rotational constants. They show that V_2' < V_2'', and that the internal rotation axis is nearly parallel to the a-axis of the cluster. If it is assumed that the structure of the rotor part does not change upon electronic excitation, the internal motion becomes simply a rotation of the water molecule around its symmetry axis. Assuming this motion, barriers of 180 cm-1 and 130 cm-1 could be estimated for the S0 and S1 states, respectively. The analysis of the rotational constants of the cluster yielded an O--O distance of the hydrogen bond of 2.93 angstrom in the ground state and 2.89 angstrom in the electronically excited state. In the equilibrium structure of the cluster, the plane containing phenol bisects the plane of the water molecule.(ref)

3-methylindole and 5-methylindole

The rotationally resolved UV excitation spectra of the S1(1Lb)<--S0 origin bands of 3-methylindole and 5-methylindole have been measured and analyzed. As a result of an internal rotation of the methyl group, each spectrum consists of rotational lines of overlapping 0a1<--0a1 and 0e<--0e torsional transitions. Like indole, 3-methylindole and 5-methylindole undergo axis reorientation upon electronic excitation. The Hamiltonian used to describe all observed spectral includes a pure rotational part, a pure torsional part, and terms describing the interaction between the internal rotation and the overall rotation. It also accounts for the axis reorientation effect. Values for the barrier heights of the methyl torsion, the angle of the methyl top axis with the inertial axes, and the rotational constants are obtained for both the S0 and the S1 state. From an analysis of the intensities of the rotational transitions, the direction of the transition moment and the axis reorientation angle are obtained. Due to quantum interference effects in the 5-methylindole spectrum the sign of these angles could be determined. (ref)

o-fluorophenol

The electronic transitions of o-fluorophenol situated at 36799.382 cm-1 and 36906.710 cm-1, denoted the A and B bands, respectively, have been investigated by high resolution fluorescence excitation spectroscopy. Hole burning studies together with the high resolution spectroscopy results show that both bands originate in the same ground state and can be fitted to the rotational constants of the cis isomer. The rotational constants for the excited states are found to be A = 3231.795 MHz, B = 2207.92 MHz and C = 1313.97 MHz for the A band and A = 3226.945 MHz, B = 2211.24 MHz and C = 1321.03 MHz for the B band. The planarity of the ground state is lost upon electronic excitation, which enhances the activity of an out-of-plane vibration. The A and B band transitions arise from excitations to respectively the zero and first overtone levels in the double-minimum potential of this out-of-plane vibration, which shows similarities to the so-called butterfly mode observed in other benzene derivatives.(ref)

4-aminobenzonitrile

The rotationally resolved fluorescence excitation spectrum of the 0-0 band in the S1 <-- S0 transition of 4-aminobenzonitrile (ABN) was recorded, at 299 nm, by using laser induced fluorescence in a molecular beam apparatus. This spectrum exhibits pure b-type character, which indicates that the electronic transition moment vector is oriented along the short molecular axis. The rotational constants of the S0 and S1 states were determined. In addition, the rotationally resolved fluorescence excitation spectra of two vibronic bands in the S1 state, at 807 and 816 cm-1, were recorded. The molecular structure of the ABN molecule is discussed by comparing the rotational constants and the inertial defects.(ref)

Benzoic acid dimer

High resolution ultraviolet spectroscopy has been used to investigate the rotationally resolved excitation spectrum of the first singlet-singlet transition in the benzoic acid dimer. The measured spectrum consists of two overlapping components. The corresponding lines in the two components are shown to originate in different levels of the ground state potential separated by a tunneling splitting produced by concerted proton exchange between the two subunits forming the dimer. The frequency separation between the two components is equal to the difference between the tunneling splittings in the ground and the excited electronic state. This frequency separation is found to be 11077 MHz. From the analysis, it is estimated that the barrier for proton tunneling changes by about 20% upon electronic excitation. The structure of the dimer in the ground state is determined to be linear, while in the excited S1 state it is slightly bent (3.41.7).(ref)