was approximately 1%; this yield U~S P. Wild?included the solid angle of collection, optical losses, and the quantum efficiency of the photomultiplier (15). This A,,, was several Two-photon fluorescence excitation spectra of single diphenyloctatetraene molecules times higher than under one-photon excitrapped in an n-tetradecane matrix were measured at cryogenic temperatures. The purely tation, because the laser wavelength is electronic zero-phonon line (transition at 444 nanometers) of these single molecules with twice the transition wavelength and the a width of about 60 megahertz was excited by a continuous-wave, single-mode laser at whole luminescence spectrum of DPOT in-888 nanometers. Even though the two-photon absorption cross section is extremely cluding the strongest 0-0 line can be colsmall, a high photon count rate and low background allowed nonlinear spectroscopy to lected without interference from stray light. be extended to the single-molecule level. This experiment also suggests the possibilityThe fluorescence excitation spectra were of two-photon single-molecule scanning microscopy.recorded as a function of the laser wavelength. All measurements were performed in supetfluid He at 1.7 K.A two-photon excitation spectrum of Optical experiments with single quantum minescence spectra of DPOT in TD (Fig. the inhomogeneous band (Fig. 3) had a systems (SQSs) have been steadily progress-I), where a small peak at 444 nm corre-maximum intensity at about 888 nm and aing. Single atoms in a beam, single ions in a sponding to the one-photon transition to S1 linewidth of about 120 GHz. The average Paul trap (I), single molecules on a surface is observed. In a solid matrix, the observed intensity of the band was proportional to (2), and single atoms in a magnetooptical lifetime T, of the S, state is about 6 ns, the square of the laser intensity and was trap (3) are promising systems that can be whereas the radiative lifetime Tmd is 66 ns roughly proportional to the concentration used to test quantum physics. Since 1989, (13). The blue luminescence ( Fig. 1) was of DPOT at room temperature (16). The optical spectroscopy of single molecules in used in this work for SM detection. peak position is in agreement with the solids (4, 5) has been achieved by several We prepared the sample by dissolving a value observed in the low-resolution specgroups (6).small amount of solid 1,8-diphenyl-1,3,5,7-tra (Fig. I), and the band shape is domi-In all of these previous experiments, the octatetraene (Aldrich) in TD (Fluka) at a nated by fluctuations called statistical fine SQS absorbed only one photon to undergo concentration of 2.5 x M at room structure (SFS) (17). This SFS is caused a transition from its ground state to the temperature. The solution was placed be-by fluctuations in the number of molecules excited state, that is, the interaction of the tween two microscope cover-glass plates and per frequency interval in the excited vol-SQS and the electromagnetic wave was lin-was rapidly cooled to liquid He temperature ume and i...

A new method based on the calculation of autocorrelation functions for spectra measured at a high acquisition rate is developed to study spectral dynamics of single molecules. The technique allows for spectroscopy with time resolutions down to the luminescence lifetime. The method is used to study spectral diffusion in two-photon excitation spectra of diphenyloctatetraene molecules doped in an n-tetradecane crystal matrix. The diffusion is light induced, and is absent in one-photon excitation spectra. It has a "steplike" time behavior, different from gradual diffusion observed in glasses.[ S0031-9007(98) Time resolution in SM spectroscopy is also desirable, for example, to study SM interactions with an environment which cause time dependent resonance frequency changes or so-called spectral diffusion (SD). According to the two-level systems (TLSs) model, which was suggested for amorphous solids [13,14] to explain their acoustic and thermodynamic properties, expanded later to include optical phenomena [15], and confirmed in many experiments [16,17], the environment is imitated by a set of TLSs with flip rates distributed from microhertz to gigahertz. A study of fast dynamics requires high time resolution, but it has been considered impossible to implement high time resolution spectroscopy for SMs so far, because the number of detected photons emitted by a SM is small and to first approximation fluctuates according to a Poisson distribution. Even when the exciting laser power saturates the transition and the emission rate is at maximum, a SM signal rarely reaches 10 5 counts͞s (a strong emitter is terrylene [18]). To measure the SM linewidth, the laser intensity should be well below the saturation value. So, only 10 4 counts͞s are detected. To determine the line shape, the recording time should be on the order of 10 ms in the best case. For molecules with 100 times lower emission rate [pentacene, diphenyloctatetraene (DPOT), and many others], this time can be as long as a few seconds. A conventional photon correlation technique [19,20] allows one to gain insight into fast SM dynamics but does not provide spectral information.In this paper, a new approach to SM spectroscopy is reported which pushes the time resolution far below one second, even for molecules with poor emission rates. This technique, which we call intensity-time-frequency correlation (ITFC) SM spectroscopy, can yield microsecond or even better time resolution with an intrinsic theoretical limit at the luminescence lifetime. The ITFC technique is used for studying dynamics in two-photon excitation (TPE) spectra of DPOT molecules in an n-tetradecane matrix [21], where a significant difference between the linewidths in one-photon excitation (OPE) and TPE spectra was observed and tentatively explained by SD induced by the powerful laser illumination required for TPE [22,23]. This explanation agrees with the significant line broadening observed for OPE data in the presence of infrared illumination [24].ITFC spectroscopy works as follows. Instead of record...

A light induced frequency shift of the 0-0 line was measured in two-photon excitation spectra of single diphenyloctatetraene molecules doped in a crystal matrix. The shifts were proportional to the laser power with a slope of about 600 MHz͞W when the laser beam of about 300 mW power was focused to a diameter of 2 mm. Significantly, the observed line broadenings were an order of magnitude smaller than the shifts. The effect is ascribed mainly to a "fast" energy exchange between a local vibration and thermal phonons created by the third harmonic C-H band absorption in the matrix, and partially to an ac Stark shift. [S0031-9007(96)

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Articles you may be interested inElectronic spectra of azaindole and its excited state mixing: A symmetry-adapted cluster configuration interaction study J. Chem. Phys. 143, 204304 (2015); 10.1063/1.4935578The elusive S 2 state, the S 1/S 2 splitting, and the excimer states of the benzene dimer J. Chem. Phys. 142, 234306 (2015) One-and two-photon excitation spectra, as well as absorption and emission spectra of diphenyloctatetraene ͑DPOT͒ in n-alkanes are investigated at low temperatures. For DPOT in n-octane we report on the measurements of one-photon excitation and emission spectra and for DPOT in n-tetradecane ͑TD͒ on the measurements of one-and two-photon excitation spectra and emission spectra. The spectra are governed by the transitions between the electronic ground (S 0 ) and the two lowest electronic excited singlet states (S 1 ,S 2 ). The interpretation is based on allowed transitions and transitions induced by the S 1 -S 2 coupling due to Herzberg-Teller promoting modes or due to static lattice-induced distortions of DPOT. A single noncentrosymmetric site is observed for DPOT in n-octane. For DPOT in TD a centrosymmetric and a noncentrosymmetric site are reported. The analysis indicates that there is a dynamical equilibrium in the population of these two sites. The experimental data are quantitatively studied by comparison with simulated spectra. The simulation calculations are based on the coupling between nonadiabatic S 1 and S 2 states, harmonic modes, and suitable transition moments and line-shape functions. For DPOT in TD the calculations reveal an interesting interference pattern in the vibronic progressions observed in two-photon excitation spectra.