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Defects in monolayer transition-metal dichalcogenides (TMDCs) may lead to unintentional doping, charge-carrier trapping, and nonradiative recombination. These effects impair electronic and optoelectronic technologies. Here we show that charged defects in MoS

Recent progress in the fluorescence detection of individual molecules [1-8] suggests that a single dye molecule can be a useful tool to probe chemical identity and activity. Measurement of fluorescence lifetime [5,6] and spectrum [6] can be augmented by knowledge of molecular orientation using polarized light [3], and triplet [2] and photoisomer excitation, as well as diffusion processes, via fluorescence-intensity correlation. Applications of fluorescent probes include the study of the dynamic conformation of membrane-bound proteins, transport of and signaling by messenger molecules, and the optical detection of the sequence of DNA. While molecules can be spatially located using near-field microscopy [5-8], near-field probes can perturb the molecule under study. We show here that molecular properties can be determined easily and in a non-perturbative manner using far-field illumination, and we obtain unperturbed spectral and lifetime data that cannot be extracted from an ensemble measurement.

The 1933 A ArF excimer laser has been used to directly time resolve the vibrational relaxation of 14N16O, 15N16O, and 14N18O into the v=0 B 2Π1/2 and v=0 a 4Π5/2 states in solid Ar host. A weak ’’conventional’’ istope effect is observed, with the smaller vibrational quanta tending to relax faster. However, the rates do not strictly correlate with either the isotopic reduced mass or the asymmetry of vibration. The host lattice phonons may directly accept energy during these relaxation processes. Intersystem crossing between v=0 B 2Π and v=8 a 4Π is shown to involve the individual spin orbit components of a 4Π.

The 1933 A ArF excimer laser has been used to excite NO 3sσ Rydberg fluorescence in solid rare gases. A comparison of Rydberg and valence spectra indicates that Rydberg fluorescence is particularly sensitive to the nature of the local environment. The data demonstrate the limits of a continuum host approximation in the small ’’bubble’’ model. All observed emission is vibrationally relaxed. The relaxation rates appear to be many orders of magnitude faster than observed for ground state diatomics with equivalent vibrational quanta.

The time dependence of Na(32P) fluorescence produced by pulsed, far‐ultraviolet photodissociation of NaI vapor at 600°C has been directly observed via the method of single‐photon counting. The experimental apparatus and procedure are described. Sodium iodide molecules photodissociate in a time much less than 1 nsec, and the fluorescence decay is exponential with τ = 16.0 ± 0.2 nsec. No evidence of collision‐induced fluorescence is seen, and quenching of the fluorescence by NaI vapor itself is not observed. Quenching by iodine vapor is studied as a function of pressure, and I2 molecules are shown to be at least 30 times more effective quenchers than I atoms. The velocity dependence of the I2 cross section in the range ∼ 12 − 25 × 104 cm/sec is studied by varying the photodissociation pulse wavelength. The fluorescence of Na(32P) following photodissociation of NaBr and of Tl(72S) following photodissociation of TlI are also observed. An average cross section for the quenching of Na(32P) by Br2 is reported. B...

The theoretical structure of transition dipole moments in extreme Renner effect molecules is considered. Pople and Longuet‐Higgins vibronic wavefunctions, which explicitly correlate nuclear and electronic motion, are employed. Constructive and destructive interference of wavefunctions causes the transition moment to vary, as a function of K and Λ, between the molecular b and ĉ axes. A few unassigned lines in the CH2(1A1 →1B1) spectrum are tentatively identified as type b transitions. It is concluded that in the dihydride radicals CH2, NH2, and BH2, l is decoupled from Λ at the large bending angle where A1 ↔ B1 transitions occur.

Time resolved Raman scattering studies of the surface reactions of organic molecules adsorbed on colloidal semiconductor (TiO2 and CdS) crystallites, following optical excitation of the crystallite, show that single electron redox reactions occur.1 The vibrational structures of the initial transient species show negligible distortion due to adsorption on the crystallite surface. TEM micrographs of the CdS crystallites (zinc blende cubic) directly image the crystallite internal lattice. The crystallites are essentially excised fragments of the corresponding bulk lattices. As diameter decreases in the range 50 to 20Å, CdS and ZnS crystallites show an increasing blue shift of the optical absorption edge (exciton peak), by as much as 0.8 eV.2,3 The exciton peak intensity also increases relative to the above gap absorption, as it shifts to higher energy. CdS crystallites of = 45Å average dimeter show corresponding exciton shifts in the LO phonon Raman excitation spectra, and the relaxed fluorescence spectra. We interpret these observations as quantum size effects. That is, these crystallites are too small for the bulk band gap to completely form. The size dependence of the crystallite lowest excited state is modelled, without adjustible parameters, at the same level of approximation as in the Wannier bulk exciton Hamiltonian.4 The exact Coulomb interaction between an electron and hole in a polarizable small crystallite is derived. The size dependences of the ionization potential and chemical redox potentials are also modelled.5 Materials with strong chemical bonds involving the valence electrons, and hence small effective masses for holes and electrons at the band edges, should show significant quantum size effects.