Two-photon spectroscopy of dipole-forbidden transitions

We have investigated the applicability of CNDO/S-type methods for the calculation of optical spectra of molecules with the special implication that the calculations should not only describe the intense, dipole-allowed transitions which dominate the one-photon absorption spectrum but also those transitions which are one-photon forbidden in first-order approximation. We show that such a method is well suited to predict dipole allowed and dipole forbidden transitions at a similar level of accuracy if double excited configurations are taken into account. In spite of the lack of perfect pairing in NDO methods there are still two types of states which exhibit a different sensitivity towards correlation effects. Therefore, the approximation by which we describe the R-dependence of the Coulomb repulsion gains much more importance than in cases where mainly dipole allowed transitions are of interest. These findings confirm results obtained earlier from theories for which the pairing theorem is valid. The calculated data show an excellent stability with respect to further increase of the number of configurations if at least about 200 energy selected configurations are taken into account.     

Two-photon spectroscopy of cyclometalated iridium complexes

The near-infrared two-photon absorption (TPA) spectra of a series of cyclometalated iridium complexes have been measured. These complexes exhibit moderately large TPA cross-sections of approximately 20 GM at the biological relevant wavelength of 800 nm. A new complex has been designed and synthesised, and found to have an increased cross-section of 44 GM at 800 nm. Full photophysical characterisation of this complex is presented.     

Two-photon MPI spectroscopy of alkyl iodides

The two-photon resonant multiphoton ionization (MPI) spectra of methyl iodide, methyl iodide-d₃, ethyl, propyl, and butyl iodide are reported in the 49 000-55 000 cm^?1 region. Four separate transitions to excited states labeled Δ, Π, Σ, Π in increasing energy are expected in this range which result from the excitation of an iodine 5pπ electron to the 6s molecular Rydberg orbital. Two-photon spectroscopy with its different selection rules and unique dependence on the laser polarization is shown to significantly advance the understanding of these transitions. In particular, laser polarization studies identify a state which is strongly two-photon allowed but absent in the UV absorption spectrum as the Σ state. Rotational contours indicate a large geometry change takes place in this transition. The two Π states appear strongly in both the one-and two-photon spectrum. Polarization analysis confirms their electronic symmetry assignment in addition to distinguishing vibronic bands arising from nontotally symmetric vibrations. No evidence is found for the Δ state in the multiphoton ionization spectrum, due to either a small two-photon cross section or a low probability of ionization following the initial two-photon transition. Further complications and characteristics of single laser MPI spectroscopy in the study of two-photon absorption in methyl iodide and other fundamental molecules are discussed.     

Dielectric spectroscopy of some heteronuclear amino alcohol complexes

The temperature dependent dielectric spectroscopic properties of two heteronuclear complexes of monoethanolamine (MEA) at a wide temperature range (303-413 K) were investigated by impedance spectroscopy, in the frequency range from 100 Hz to 100 kHz. The frequency dependence of the impedance spectra plotted in the complex plane shows semi-circles. The Cole-Cole diagrams have been used to determine the molecular relaxation time, tau. The temperature dependence of tau is expressed by thermally activated process. Relaxation frequencies corresponding to the rotation of the molecules about their long axes are expected to lie above 10 MHz and exhibit Arrhenius behavior, where a single slope is observed with activation energy values equal to 0.67 and 0.78 eV. The ac conductivity sigma(ac) (omega) is found to vary as omega(s) with the index s相似文献   

Improved LeRoy-Bernstein near-dissociation expansion formula, and prospect for photoassociation spectroscopy

NDE (Near-dissociation expansion) including LeRoy-Bernstein formulas are improved by taking into account the multipole expansion coefficients and the nonasymptotic part of the potential curve. The theory is tested with the Rydberg-Klein-Rees (RKR) potential curve of the Cs(2)(0(g) (-)6s+6p(3/2)) state. Results indicate that the formula could be used to improve the determination of the asymptotic coefficient (within a 1% accuracy) and to extract relativistic correction from photoassociation spectra of long-range potential curve of diatomic molecules.     

Two-photon microscopy and spectroscopy of lanthanide bioprobes.

The linear and non-linear photophysical properties of tris-dipicolinate europium and terbium complexes (absorption, emission, lifetime, luminescence induced by two-photon absorption) are studied in the crystalline state as well as in protein derivative crystals and compared to those in solution. Upon laser irradiation at 532 nm, luminescence of terbium is induced by a two-photon antenna effect, whereas luminescence of europium results from one-photon absorption in forbidden f-f transitions. Finally, linear and two-photon microscopy imaging experiments on biological and bio-inspired crystals are performed. These first proof-of-concept experiments open the way for the development of time-resolved non-linear microscopy that should combine the advantages of lanthanide luminescence (long lifetime, sharp emission bands, insensitivity to oxygen) with those of confocal biphotonic excitation (near-IR excitation, 3D resolution and reduced photodamage).     

Indirect detection of magnetic resonance by heteronuclear two-dimensional spectroscopy

A new technique is proposed to indirectly record with high sensitivity the nuclear magnetic resonance of nuclei with a low gyromagnetic ratio. The method relies on a coherent transfer of transverse magnetization to nuclei of high gyromagnetic ratio.     

Natural abundance nitrogen-15 NMR by enhanced heteronuclear spectroscopy

The detection of NMR spectra of less sensitive nuclei coupled to protons may be significantly unproved by a two-dimensional Fourier transform technique involving a double transfer of polarization. The method is adequate to obtain natural abundance ¹⁵N spectra in small sample volumes with a commercial spectrometer     

A new experimental method for heteronuclear correlation spectroscopy with cross-polarization

A new experiment is proposed for two-dimensional heteronuclear correlation spectroscopy. It involves the rapid creation of pure heteronuclear ZQC under cross-polarization, followed by an evolution period during which ZQC is converted to DQC. A read pulse is used to detect the signal. No phase cycling is required to determine the sign of ω₁.     

Two-photon excitation fluorescence correlation spectroscopy of diffusion for Gaussian-Lorentzian volumes

Fluorescence correlation spectroscopy (FCS) is valuable in many scientific domains where diffusion plays a fundamental role. One important experimental realization is based on fluorescence induced by two-photon excitation (TPE). In comparison with one-photon excitation (OPE), TPE-FCS defines better the interrogation volume and the background noise is sensibly reduced. Within this context and for overfilled objective lenses, the three-dimensional Gaussian (3DG) approximation, according to which the spectroscopic interaction is spatially defined by Gaussian profiles only, guarantees a simple analytical data interpretation. By contrast, the volume illuminated by the laser beam focused with partially filled objective lenses follows a Gaussian-Lorentzian (GL) distribution that is taken into account by means of numerical methods only. Here we show that contrary to common belief, the assumption of a GL volume does not hamper analytical treatment of TPE-FCS. Differences and similarities in comparison with the 3DG approximation are discussed.     

Applications of heteronuclear NMR spectroscopy in biological and medicinal inorganic chemistry

There is a wide range of potential applications of inorganic compounds, and metal coordination complexes in particular, in medicine but progress is hampered by a lack of methods to study their speciation. The biological activity of metal complexes is determined by the metal itself, its oxidation state, the types and number of coordinated ligands and their strength of binding, the geometry of the complex, redox potential and ligand exchange rates. For organic drugs a variety of readily observed spin I = 1/2 nuclei can be used (¹H, ¹³C, ¹⁵N, ¹⁹F, ³¹P), but only a few metals fall into this category. Most are quadrupolar nuclei giving rise to broad lines with low detection sensitivity (for biological systems). However we show that, in some cases, heteronuclear NMR studies can provide new insights into the biological and medicinal chemistry of a range of elements and these data will stimulate further advances in this area.     

Relativistic calculations of ground and excited states of LiYb molecule for ultracold photoassociation spectroscopy studies

We report a series of quantum-chemical calculations for the ground and some of the low-lying excited states of an isolated LiYb molecule by the spin-orbit multistate complete active space second-order perturbation theory (SO-MS-CASPT2). Potential energy curves, spectroscopic constants, and transition dipole moments (TDMs) at both spin-free and spin-orbit levels are obtained. Large spin-orbit effects especially in the TDMs of the molecular states dissociating to Yb((3)P(0,1,2)) excited states are found. To ensure the reliability of our calculations, we test five types of incremental basis sets and study their effect on the equilibrium distance and dissociation energy of the ground state. We also compare CASPT2 and CCSD(T) results for the ground state spectroscopic constants at the spin-free relativistic level. The discrepancies between the CASPT2 and CCSD(T) results are only 0.01 ? in equilibrium bond distance (R(e)) and 200 cm(-1) in dissociation energy (D(e)). Our CASPT2 calculation in the supermolecular state (R=100 a.u.) with the largest basis set reproduces experimental atomic excitation energies within 3% error. Transition dipole moments of the super molecular state (R=100 a.u.) dissociating to Li((2)P) excited states are quite close to experimental atomic TDMs as compared to the Yb((3)P) and Yb((1)P) excited states. The information obtained from this work would be useful for ultracold photoassociation experiments on LiYb.     

Formation of ultracold SrYb molecules in an optical lattice by photoassociation spectroscopy: theoretical prospects

State-of-the-art ab initio techniques have been applied to compute the potential energy curves for the SrYb molecule in the Born-Oppenheimer approximation for the electronic ground state and the first fifteen excited singlet and triplet states. All the excited state potential energy curves were computed using the equation of motion approach within the coupled-cluster singles and doubles framework and large basis-sets, while the ground state potential was computed using the coupled cluster method with single, double, and noniterative triple excitations. The leading long-range coefficients describing the dispersion interactions at large interatomic distances are also reported. The electric transition dipole moments have been obtained as the first residue of the polarization propagator computed with the linear response coupled-cluster method restricted to single and double excitations. Spin-orbit coupling matrix elements have been evaluated using the multireference configuration interaction method restricted to single and double excitations with a large active space. The electronic structure data were employed to investigate the possibility of forming deeply bound ultracold SrYb molecules in an optical lattice in a photoassociation experiment using continuous-wave lasers. Photoassociation near the intercombination line transition of atomic strontium into the vibrational levels of the strongly spin-orbit mixed b(3)Σ(+), a(3)Π, A(1)Π, and C(1)Π states with subsequent efficient stabilization into the v' = 1 vibrational level of the electronic ground state is proposed. Ground state SrYb molecules can be accumulated by making use of collisional decay from v' = 1 to v' = 0. Alternatively, photoassociation and stabilization to v' = 0 can proceed via stimulated Raman adiabatic passage provided that the trapping frequency of the optical lattice is large enough and phase coherence between the pulses can be maintained over at least tens of microseconds.     

Two-photon spectroscopy of the low-lying singlet states of naphthalene and acenaphthene

Two-photon excitation spectra of naphthalene and acenaphthene have been measured up to 50000 cm^?1. In naphthalene. three two-photon allowed states are observed for which the symmetry assignment is confirmed by polarization. The corre- sponding transitions are also seen in acenaphthene. The experimental data are in excellent agreement with theoretical predictions.     

Fast measurement of heteronuclear relaxation: frequency-domain analysis of NMR accordion spectroscopy

NMR relaxation parameters are usually derived from series of 2D experiments. The whole procedure can be very time consuming, especially for the study of the relaxation of nuclei at natural abundance. Palmer and Mandel have proposed the use of accordion spectroscopy to determine one relaxation parameter using two experiments only. In this paper, we show that the experimental time can be further reduced, by recording only three experiments for the determination of both the longitudinal and transverse relaxation rates. The analysis of the experiments is performed in the frequency domain, the relaxation rates being deduced from the linewidth of the peaks of interest. A detailed statistical analysis of errors introduced by the line fitting procedure on derived relaxation parameters was used to derive guidelines for the choice of experimental parameters. This procedure was applied to the study of the Cα relaxation parameters of a six-residue unlabeled peptide. The results were compared with those obtained by classical accordion spectroscopy. Copyright © 2001 John Wiley & Sons, Ltd.     

Two-photon fluorescence spectroscopy of individually trapped pseudoisocyanine J-aggregates in aqueous solution

We have investigated a pseudoisocyanine dye aqueous solution including nanometer-sized J-aggregates by combining optical trapping and two-photon fluorescence spectroscopy. By focusing an intense near-infrared laser into an 8 x 10(-3) M solution, the intense fluorescence from J-aggregates for a few to tens of seconds is observed intermittently, indicating that individual J-aggregates are trapped in and diffuse out from a focal spot. The peak position and full width at half-maximum of the J-band are different from each other. By measuring 171 J-aggregates, it was found that J-aggregates can be classified largely into two groups. The existence of two kinds of groups of J-aggregates could be attributed to the difference in the nucleation process, which is affected by the substrate. J-aggregates possessing a J-band of a narrower bandwidth in a shorter wavelength region are trapped for a longer period of time, indicating that highly ordered J-aggregates are trapped for a longer period of time because of their high polarizability.     

Two-photon absorption spectroscopy of iodine monochloride in the 5 eV region

Two-photon high resolution sequential spectroscopy has been used to excite iodine monochloride from X¹Σ⁺ ground state to the intermediate A³Π₁ state and thence to a final electronic state at 4.82 eV. Vibrational and rotational analyses of this state have been carried out for both isotopic species. For I³⁵Cl, T_e = 38916.0 cm^?1 ω_e = 168.99 cm^?1, ω_ex_e = 0.357 cm^?1 and B_e = 0.05685 cm^?1. The state probably has Ω = 1 in case (c) coupling approximation. It is also shown how to two-photon technique enables rotational line structure of the A ← X transition to be selectively excited for either isotopic species at a resolution of 500000, from an absorption mixture containing natural iodine monochloride plus its iodine dissociation product at equilibrium vapour pressure.     

Methyl proton contacts obtained using heteronuclear through-bond transfers in solid-state NMR spectroscopy

A two-dimensional proton-mediated carbon-carbon correlation experiment that relies on through-bond heteronuclear magnetization transfers is demonstrated in the context of solid-state NMR of proteins. This new experiment, dubbed J-CHHC by analogy to the previously developed dipolar CHHC techniques, is shown to provide selective and sensitive correlations in the methyl region of 2D spectra of crystalline organic compounds. The method is then demonstrated on a microcrystalline sample of the dimeric protein Crh (2 x 10.4 kDa). A total of 34 new proton-proton contacts involving side-chain methyl groups were observed in the J-CHHC spectrum, which had not been observed with the conventional experiment. The contacts were then used as additional distance restraints for the 3D structure determination of this microcrystalline protein. Upon addition of these new distance restraints, which are in large part located in the hydrophobic core of the protein, the root-mean-square deviation with respect to the X-ray structure of the backbone atom coordinates of the 10 best conformers of the new ensemble of structures is reduced from 1.8 to 1.1 A.