Electronic spectroscopy of nonalternant hydrocarbons inside helium nanodroplets

We have recorded the electronic spectra of three polycyclic aromatic hydrocarbons (acenaphtylene, fluoranthene, and benzo(k)fluoranthene) containing a five-member ring and their van der Waals complexes with argon and oxygen with a molecular beam superfluid helium nanodroplet spectrometer. Although the molecules, which differ by addition of one or two fused benzene rings to acenaphtylene, have the same point group symmetry, the spectral lineshapes show distinct differences in the number of zero phonon lines and shapes of the phonon wings. Whereas the smallest molecule (acenaphtylene) has the most complicated line shape, the largest molecule (benzo(k)fluoranthene) shows different lineshapes for different vibronic transitions. The van der Waals complexes of fluoranthene exhibit more peaks than the theoretically allowed number of isomeric complexes with argon/oxygen. The current models of molecular solvation in liquid helium do not adequately explain these discrepancies.     

Probing collective excitations in helium nanodroplets: observation of phonon wings in the infrared spectrum of methane

The authors have recorded the nu(3) infrared spectrum of methane in helium nanodroplets using our cw infrared optical parametric oscillator. In a previous paper, Nauta and Miller [Chem. Phys. Lett. 350, 225 (2001)] reported the observation of the monomer rovibrational transitions of methane in helium nanodroplets. Here, they report the observation of additional absorption bands in the frequency range between 2990 and 3070 cm(-1) blueshifted compared to the monomer transitions. They attribute these absorption features to phonon wings of individual rovibrational transitions, i.e., the simultaneous excitation of collective excitation modes of the quantum fluid and the rovibrational excitation of the methane monomer in the helium nanodroplet.     

Laser induced fluorescence of Mg-phthalocyanine in He droplets: evidence for fluxionality of large H2 clusters at 0.38 K

The formation of Ar and H2 clusters, having up to 900 particles in helium droplets, has been studied via laser induced fluorescence of attached Mg-phthalocyanine (Mg-Pc) molecules. In the experiments, one Mg-Pc molecule in average was added to each He droplet either before or after the cluster species, and the shift of the spectrum of the Mg-Pc molecules was studied as a function of the cluster size. For Ar clusters, about a factor of 2 smaller matrix shift was observed for the late pickup of the Mg-Pc molecules as compared with the prior pickup, indicating that in the former case, the Mg-Pc molecules reside on the surface of the preformed Ar clusters. On the other hand, the spectra of the Mg-Pc molecules attached to H2 clusters are independent of the pickup order, which is consistent with Mg-Pc molecules residing near the center of the H2 clusters in both cases. Therefore H2 clusters remain fluxional in helium droplets at T=0.38 K. No significant differences in the spectra were observed between the para-H2 and ortho-H2 clusters.     

Electronic polarization spectroscopy of metal phthalocyanine chloride compounds in superfluid helium droplets

We report the electronic polarization spectroscopy of two metal phthalocyanine chloride compounds (MPcCl, M=Al,Ga) embedded in superfluid helium droplets and oriented in a dc electric field. For both compounds, the laser induced fluorescence spectra show preference for perpendicular excitation relative to the orientation field. This result indicates that the permanent dipoles of both compounds are predominantly perpendicular to the transition dipole. Since the permanent dipole derives from the metal chloride, while the transition dipole derives from the phthalocyanine chromophore, in the plane of phthalocyanine, this qualitative result is not surprising. However, quantitative modeling reveals that this intuitive model is inadequate and that the transition dipole might have tilted away from the molecular plane of phthalocyanine. The out of plane component of the transition dipole amounts to approximately 10% if the permanent dipole is assumed to be approximately 4 debye. The origin for this tilt is puzzling, and we tentatively attribute it to the transition of nonbonding orbitals, either from the chlorine atom or from the bridge nitrogen atom, to the pi* orbitals of the phthalocyanine chromophore. On the other hand, although unlikely, we cannot completely exclude the possibility that both our high level density functional theory calculation and ab initio results severely deviate from reality. The droplet matrix induces redshifts in the origin of the electronic transition and produces discrete phonon wings. Nevertheless, in dc electric fields, all phonon wings and the zero phonon line demonstrate the same dependence on the polarization direction of the excitation laser. Although electronic excitation does couple to the superfluid helium matrix and the resulting phonon wings add complications to the electronic spectrum, this coupling does not affect the direction of the electronic transition dipole. Electronic polarization spectroscopy in superfluid helium droplets is thus still informative in revealing the permanent dipole and its relation relative to the transition dipole.     

Structure and dynamics of phthalocyanine-argonn (n = 1-4) complexes studied in helium nanodroplets

Van der Waals clusters of phthalocyanine with 1-4 argon atoms formed inside superfluid helium nanodroplets have been investigated by recording fluorescence excitation spectra as well as emission spectra. The excitation spectra feature a multitude of sharp lines when recorded in superfluid helium droplets in contrast to the respective spectra measured in a seeded supersonic beam (Cho et al. Chem. Phys. Lett. 2000, 326, 65). The pickup technique used for doping of the phthalocyanine and the argon into the droplets allows for nondestructive analysis of the cluster sizes. Alternation of the pickup sequence gives information on the binding site of the argon atoms. The investigation of dispersed emission spectra in helium droplets can be used as a special tool for the identification of 0(0)0 transitions within the variety of sharp lines seen in the excitation spectra. Thus, different isomers of the clusters can be distinguished. Moreover, the emission spectra reveal information on dynamic processes such as vibrational predissociation of the van der Waals complexes and interconversion among isomeric species. The binding energy of the phthalocyanine-argon1 complex in helium droplets was estimated to be at most 113 cm-1.     

Effects of inhomogeneity and site selective impurity—phonon coupling In solid solutions

Inhomogeneously broadened fluorescene spectra of low-temperature impurity molecules are considered. It is demonstrated that the dependence of the spectra on monochromatic excitation frequency is caused first of all by different ratios of the molecules excited in no-phonon lines to those excited in phonon wings at different frequencies. There is no need to suppose any drastic alteration of the electron—phonon coupling. A possibility of testing the existence of such an alteration is supposed.     

Electronic spectroscopy of biphenylene inside helium nanodroplets

We have recorded the S1 <-- S0 electronic spectra of Biphenylene and its Ar and O2 van der Waals complexes inside helium nanodroplets using beam depletion detection. In general, the spectrum is similar to the previously reported high-resolution REMPI spectrum. The zero phonon lines, however, are split similar to the previously reported tetracene case. The calculated potential energy surface predicts that helium atoms can simultaneously occupy all equivalent global minima positions. Therefore, it appears that the splitting cannot be explained either by different isomers or by tunneling. Furthermore, surprisingly the splitting is retained for the Ar van der Waals complexes (and possibly for the O2 complex as well). This case suggests that the current models of the origin of zero phonon line splitting and the helium solvation are incomplete.     

The excited states and vibronic spectroscopy of diphenyldiacetylene and diphenylvinylacetylene

Laser induced fluorescence (LIF) excitation scans and dispersed fluorescence (DFL) spectra have been recorded for two four-carbon α,ω-diphenyl systems, diphenyldiacetylene (DPDA, φ-C≡C-C≡C-φ) and trans-diphenylvinylacetylene (DPVA, φ-CH≡CH-C≡C-φ) as isolated molecules cooled in a supersonic expansion. While these molecules have similar conjugation length, they exhibit strikingly different vibronic spectroscopy and photophysics. The near-UV LIF excitation spectrum of diphenyldiacetylene has its electronic origin at 32,158 cm(-1), and a strong progression in the C≡C stretch (2156 cm(-1)). All transitions are inherently broad, with widths of ~30 cm(-1) fwhm or greater. The S(1) origin DFL spectrum is composed of sharp transitions with Franck-Condon activity mirroring that in the excitation spectrum, and broad emission shifted well to the red ascribable to phosphorescence on the μs timescale. Using ab initio calculations, it is possible to show that DPDA exists as a single, planar conformer with D(2h) symmetry. In contrast, trans-diphenylvinylacetylene shows intense sharp transitions in both LIF and DFL spectra with an S(0)-S(1) origin of 31,183.2 cm(-1) and long progressions involving the in-plane fundamentals ν(53) (bridge-phenyl bending) and ν(51) (bridge-phenyl stretch). A sharp reduction in fluorescence yield in DPVA occurs within 300 cm(-1) of the S(1) origin. Possible causes for the photophysical processes occurring in the two molecules are discussed.     

Applying Unconventional Spectroscopies to the Single-Molecule Magnets,Co(PPh3)2X2 (X=Cl,Br, I): Unveiling Magnetic Transitions and Spin-Phonon Coupling

Large separation of magnetic levels and slow relaxation in metal complexes are desirable properties of single-molecule magnets (SMMs). Spin-phonon coupling (interactions of magnetic levels with phonons) is ubiquitous, leading to magnetic relaxation and loss of memory in SMMs and quantum coherence in qubits. Direct observation of magnetic transitions and spin-phonon coupling in molecules is challenging. We have found that far-IR magnetic spectra (FIRMS) of Co(PPh₃)₂X₂ ( Co-X ; X=Cl, Br, I) reveal rarely observed spin-phonon coupling as avoided crossings between magnetic and u-symmetry phonon transitions. Inelastic neutron scattering (INS) gives phonon spectra. Calculations using VASP and phonopy programs gave phonon symmetries and movies. Magnetic transitions among zero-field split (ZFS) levels of the S=3/2 electronic ground state were probed by INS, high-frequency and -field EPR (HFEPR), FIRMS, and frequency-domain FT terahertz EPR (FD-FT THz-EPR), giving magnetic excitation spectra and determining ZFS parameters (D, E) and g values. Ligand-field theory (LFT) was used to analyze earlier electronic absorption spectra and give calculated ZFS parameters matching those from the experiments. DFT calculations also gave spin densities in Co-X , showing that the larger Co(II) spin density in a molecule, the larger its ZFS magnitude. The current work reveals dynamics of magnetic and phonon excitations in SMMs. Studies of such couplings in the future would help to understand how spin-phonon coupling may lead to magnetic relaxation and develop guidance to control such coupling.     

On the role of methyl torsional modes in the intersystem crossing dynamics of isolated molecules

Rotationally resolved fluorescence excitation spectra of the 0(0)(0) bands of the S1<--S0 electronic transitions of 2- and 5-methylpyrimidine (2MP and 5MP, respectively) have been observed and assigned. Both spectra were found to contain two sets of rotational lines, one associated with the sigma=0 torsional level and the other associated with the sigma=+/-1 torsional level of the attached methyl group. Analyses of their structure using the appropriate torsion-rotation Hamiltonian yields the methyl group torsional barriers of V6'=1.56 and V6'=8.28 cm(-1) in 2MP and V6'=4.11 and V6'=58.88 cm(-1) in 5MP. Many of the lines in both spectra are fragmented by couplings with lower lying triplet states. Analyses of some of these perturbations yield approximate values of the intersystem crossing matrix elements, from which it is concluded that the sigma=+/-1 torsional levels of the S1 state are significantly more strongly coupled to the T1 state than the sigma=0 torsional levels.     

Charge transfer vibronic transitions in uranyl tetrachloride compounds

The electronic and vibronic interactions of uranyl (UO(2))(2+) in three tetrachloride crystals have been investigated with spectroscopic experiments and theoretical modeling. Analysis and simulation of the absorption and photoluminescence spectra have resulted in a quantitative understanding of the charge transfer vibronic transitions of uranyl in the crystals. The spectra obtained at liquid helium temperature consist of extremely narrow zero-phonon lines (ZPL) and vibronic bands. The observed ZPLs are assigned to the first group of the excited states formed by electronic excitation from the 3σ ground state into the f(δ,?) orbitals of uranyl. The Huang-Rhys theory of vibronic coupling is modified successfully for simulating both the absorption and luminescence spectra. It is shown that only vibronic coupling to the axially symmetric stretching mode is Franck-Condon allowed, whereas other modes are involved through coupling with the symmetric stretching mode. The energies of electronic transitions, vibration frequencies of various local modes, and changes in the O═U═O bond length of uranyl in different electronic states and in different coordination geometries are evaluated in empirical simulations of the optical spectra. Multiple uranyl sites derived from the resolution of a superlattice at low temperature are resolved by crystallographic characterization and time- and energy-resolved spectroscopic studies. The present empirical simulation provides insights into fundamental understanding of uranyl electronic interactions and is useful for quantitative characterization of uranyl coordination.     

Fluorescence and ultraviolet absorption spectra and structure of coumaran and its ring-puckering potential energy function in the S1(pi,pi*) excited state

The fluorescence excitation (jet cooled), single vibrational level fluorescence, and the ultraviolet absorption spectra of coumaran associated with its S1(pi,pi*) electronic excited state have been recorded and analyzed. The assignment of more than 70 transitions has allowed a detailed energy map of both the S0 and S1 states of the ring-puckering (nu45) vibration to be determined in the excited states of nine other vibrations, including the ring-flapping (nu43) and ring-twisting (nu44) vibrations. Despite some interaction with nu43 and nu44, a one-dimensional potential energy function for the ring puckering very nicely predicts the experimentally determined energy level spacings. In the S1(pi,pi*) state coumaran is quasiplanar with a barrier to planarity of 34 cm(-1) and with energy minima at puckering angles of +/-14 degrees. The corresponding ground state (S0) values are 154 cm(-1) and +/-25 degrees . As is the case with the related molecules indan, phthalan, and 1,3-benzodioxole, the angle strain in the five-membered ring increases upon the pi-->pi* transition within the benzene ring and this increases the rigidity of the attached ring. Theoretical calculations predict the expected increases of the carbon-carbon bond lengths of the benzene ring in S1, and they predict a barrier of 21 cm(-1) for this state. The bond length increases at the bridgehead carbon-carbon bond upon electron excitation to the S1(pi,pi*) state give rise to angle changes which result in greater angle strain and a nearly planar molecule.     

Communication: Barium ions and helium nanodroplets: Solvation and desolvation

The solvation of Ba(+) ions created by the photoionization of barium atoms located on the surface of helium nanodroplets has been investigated. The excitation spectra corresponding to the 6p (2)P(1∕2) ← 6s (2)S(1∕2) and 6p (2)P(3∕2) ← 6s (2)S(1∕2) transitions of Ba(+) are found to be identical to those recorded in bulk He II [H. J. Reyher, H. Bauer, C. Huber, R. Mayer, A. Schafer, and A. Winnacker, Phys. Lett. A 115, 238 (1986)], indicating that the ions formed at the surface of the helium droplets become fully solvated by the helium. Time-of-flight mass spectra suggest that following the excitation of the solvated Ba(+) ions, these are being ejected from the helium droplets either as bare Ba(+) ions or as small Ba(+)He(n) (n < 20) complexes.     

Spectroscopy of Cs attached to helium nanodroplets

Cesium oligomers are formed on helium nanodroplets which are doped with one or a few Cs atoms. The monomer absorption of the first electronic p<--s transition upon laser excitation is probed. Spectra employing laser-induced fluorescence, beam depletion, and resonant photoionization are compared. In particular, mass-resolved photoionization allows us to specifically probe excitation induced processes such as, e.g., the formation of cesium-helium exciplexes. Absorption spectra of Cs dimers and trimers are recorded in the spectral region accessible by a Ti:sapphire laser. Assignment of dimer spectra is achieved by comparison with model calculations based on ab initio potentials. Electronic absorption lines of Cs trimers are attributed to transitions in the quartet manifold.     

Spectroscopic investigation of the solvation of organic molecules in superfluid helium droplets

The spectroscopy of molecules doped into superfluid helium nanodroplets provides valuable information on the process of solvation in superfluid helium. In continuation of an earlier report on emission spectra of various phthalocyanines showing a splitting of all molecular transitions in the range of about 5-12 cm(-1), the emission spectra of tetracene, pentacene, and perylene in superfluid helium droplets are presented. The new spectra and the results obtained for the phthalocyanines are explained by an empirical model which accounts for the existence of different metastable configurations of a nonsuperfluid solvation layer around the guest molecule.     

S1(1A1)<--S0(1A1) transition of benzo[g,h,i]perylene in supersonic jets and rare gas matrices

The study of the S1(1A1)<--S0(1A1) transition of benzo[g,h,i]perylene (BghiP, C22H12) in supersonic jets and solid rare gas matrices is reported. In the jet-cooled spectrum, the origin band position is located at 25,027.1+/-0.2 cm-1, the assignment being supported by the analysis of vibrational shifts and rotational band contours. Except for the origin band, which is weak, all bands are attributed to the fundamental excitation of nontotally symmetric b1 vibrational modes of S1. The intensity pattern is interpreted as a consequence of the weak oscillator strength of the electronic transition combined with intensity-borrowing through vibronic interaction between the S1(1A1) and S2(1B1) states. The spectra of the S1(1A1)<--S0(1A1) and S2(1B1)<--S0(1A1) transitions have also been measured for BghiP in solid neon and argon matrices. The comparison of the redshifts determined for either transition reveals that the polarizability of BghiP is larger in its S2 than in its S1 state. Bandwidths of 2.7 cm-1 measured in supersonic jets, which provide conditions relevant for astrophysics, are similar to those of most diffuse interstellar bands. The electronic transitions of BghiP are found to lie outside the ranges covered by present databases. From the comparison between experimental spectra and theoretical computations, it is concluded that the accuracy of empirical and ab initio approaches in predicting electronic energies is still not sufficient to identify astrophysically interesting candidates for spectroscopic laboratory studies.     

Electronic spectroscopy of benzo[g,h,i]perylene and coronene inside helium nanodroplets

We have recorded the electronic spectra of benzo[g,h,i]perylene and coronene and their van der Waals complexes with argon and oxygen with a helium-nanodroplet depletion spectrometer. These molecules differ by the addition of one and two fused benzene rings to perylene, which was previously studied in helium. The coronene spectrum is similar to a previously reported jet-cooled laser-induced fluorescence (LIF) spectrum. The van der Waals complexes with argon and oxygen show different complexation sites and maximum number of adsorbants. We report a vibronically resolved benzo[g,h,i]perylene S(1) <-- S(0) spectrum. The spectral lines are split in a similar way to that of several molecules studied before. However, surprisingly, while the van der Waals complexes with argon are free of the splitting, the complexes with oxygen retain the splitting, with increased linewidth and splitting. We could also observe the S(2) <-- S(0) origin transition of benzo[g,h,i]perylene which was previously observed by cavity ring down spectroscopy. While in general the two spectra are quite similar, the relative intensities and spectral shifts of several lines are different.     

Fluorescence studies of terrylene in a supersonic jet: indication of a dark electronic state below the allowed transition

Jet-cooled terrylene has been studied in helium buffer gas using a pulsed nozzle by means of laser-induced fluorescence. Fluorescence excitation and two-color depletion experiments (resulting in hole burning spectra) are presented. Analysis of the spectra leads to the conclusion that another excited electronic state is present in the vicinity of the allowed 1B1u state. Assuming (according to previous literature suggestions Karabunarliev, S.; Baumgarten, M.; Müllen, K. J. Phys. Chem. A 1998, 102, 7029) that this dark state is the 21Ag state, we discuss the vibrational structure of the fluorescence excitation spectrum in terms of two manifolds of vibronic states belonging to Sd(21Ag) and S1(1B1u) states. The anomalous shift between excitation and dispersed fluorescence spectra observed earlier for terrylene in a neon matrix is discussed as a consequence of terrylene electronic relaxation to the low-energy dark state.     

Theoretical interpretation of electronic absorption and emission transitions in 9-acridinones

Stationary absorption, fluorescence excitation and fluorescence spectra for 9(10H)-acridinone, 9(10-methyl)-acridinone, 2-methyl-9(10-methyl)-acridinone, 2-nitro-9(10-methyl)-acridinone, 9(10-ethyl)-acridinone and 9(10-phenyl)-acridinone dissolved in 1,4-dioxane, methyl alcohol or acetonitrile, as well as the available spectral characteristics reported by others were compared with those predicted theoretically at the semi-empirical PM3/CI (including the solvent effect within the COSMO model) or PM3/S levels of theory, in order to interpret spectral features of the compounds, i.e. the energies and probabilities of S0-->Sn, S0-->T1, T1-->T2, S1-->S0, T1-->S0 and S1-->T1 transitions. Calculations at the PM3 and PM3/CI levels of theory enabled the structural changes accompanying S0-->S1, S1-->T1 and T1-->S0 transitions to be investigated; they yielded, moreover, basic physicochemical characteristics of the molecules in the ground and excited electronic states. Theoretically predicted dipole moments and charge distributions in the S0, S1 and T1 states provided further insight into the nature of electronic transitions in 9-acridinones. The predicted characteristics correlate quite well with the available experimental ones, thus providing confirmation of the utility of theory in predicting the features of electronically excited molecules and interpreting the electronic transitions occurring in them.     

Electronic spectra and crystal field analysis of energy levels of Ho3+ in HoF6(3-)

The electronic absorption spectra of single crystals of Cs(2)NaHoF(6) have been recorded in the spectral region between 4700 and 42000 cm(-1) at temperatures down to 10 K. The structure in the (5)I(8) → (5)I(J) (J = 7-4), (5)F(J) (J = 5-1), (5)S(2), (5)G(J) (J = 4-6), (3)K(J) (J = 7, 8) transitions has been analyzed and assigned. The emission spectra (5)S(2) → (5)I(J) (J = 6-8) and (5)G(4) → (5)I(J) (J = 5-7), (5)F(5) have also been recorded at 10 K for crystals of Cs(2)NaHoF(6) and partly also for samples of Cs(2)NaHoF(6):Yb(3+). The spectra comprise magnetic dipole zero phonon lines and electric dipole allowed one-phonon vibronic sidebands. From the detailed interpretation of the emission and absorption spectra, aided by a clear understanding of the vibrational behavior of the HoF(6)(3-) moiety and by magnetic dipole intensity calculations, a data set of 59 energy levels spanning 17 multiplet terms was derived. Crystal field calculations were then performed using a 4f(10) basis, as well as including the configuration interaction with a p-electron configuration. The latter calculation, which employed 14 parameters, gave better agreement with experiment and the mean deviation was 13.5 cm(-1). A comparison with the energy level fittings for Cs(2)NaHoCl(6) has been included. The crystal field parameters for the fluoro- and chloro-systems followed empirically predicted ratios.