Photoexcitation

About: Photoexcitation is a research topic. Over the lifetime, 5874 publications have been published within this topic receiving 134733 citations.

Fluorescence from extreme ultraviolet photoexcitation of CF4

Abstract: Photoabsorption and fluorescence cross sections of CF4 were measured in the 50–130 nm region using synchrotron radiation as a light source. Oscillator strengths for Rydberg states were determined from the absorption cross sections measured. The fluorescence spectra were dispersed to identify the emission species. Fluorescences appear in the excitation wavelength regions of 50–57.4, 75–80, and 85–95 nm, in which the emitters are attributed to CF+4, CF2, and CF3, respectively. The fluorescence quantum yield has been determined from the absorption and fluorescence cross sections measured simultaneously. The yield has a maximum of 26% at 55 nm.

Singlet–Triplet Absorption of Crystalline Naphthalene by High‐Resolution Photoexcitation Spectroscopy

Abstract: The high‐resolution singlet–triplet absorption spectrum of crystalline naphthalene at 4.2°K was obtained by a photoexcitation technique. The spectrum is presented and analyzed. The spectrum primarily consists of strong Ag modes that exhibit factor‐group splitting and that are primarily polarized normal to the molecular plane. The total factor‐group splitting of − 32 cm−1 is in agreement with that measured directly in absorption. The crystal polarization properties of the spectrum serve to identify the high‐energy components of the Davydov pairs as belonging to the (+) or Au factor‐group component. The spectrum clearly reveals the presence of certain antisymmetric modes, which appear with an intensity that is about two orders of magnitude less than that of the 0, 0.

Negative‐ion mass spectrometric study of ion‐pair formation in the vacuum ultraviolet. III. SF6→F−+SF+5

Abstract: Ion‐pair formation from photoexcitation of SF6 has been studied by negative‐ion mass spectrometry using synchrotron radiation in the 11.27–31.0 eV photon energy range. Negative ions F−, SF−6, and SF−5 have been observed. The appearance energy of the F− ion is about 1 eV higher than the thermochemical threshold for the formation of the pair of the ground state ions F−(1Sg) and SF+5(X1A1). The peak features observed in the F− efficiency curve are interpreted as resulting from transitions to neutral excited states with the 1T1u symmetry which effectively couple with ion‐pair states through avoided potential surface crossings. The peaks assigned to diffuse Rydberg states are distinctively enhanced in the F− efficiency curve, probably because of large transition probabilities from the dissociative Rydberg states to the ion‐pair states. In contrast, the excited states of valence type autoionize in a short period and have quite small branching to the ion‐pair channel. Consequently, the corresponding peaks are markedly suppressed in the F− spectrum. Assignments of the peak features in the previous photoabsorption spectra are also performed by using the term values for related Rydberg and virtual valence orbitals. Other negative ions observed, SF−6 and SF−5, are produced by resonance capture of low energy electrons emitted by photoionization of the parent molecules, and are not of major concern of the present study.

Excited-state absorption and ultrafast relaxation dynamics of porphyrin, diprotonated porphyrin, and tetraoxaporphyrin dication.

Abstract: The relaxation dynamics of unsubstituted porphyrin (H2P), diprotonated porphyrin (H4P2+), and tetraoxaporphyrin dication (TOxP2+) has been investigated in the femtosecond−nanosecond time domain upon photoexcitation in the Soret band with pulses of femtosecond duration. By probing with spectrally broad femtosecond pulses, we have observed transient absorption spectra at delay times up to 1.5 ns. The kinetic profiles corresponding with the band maxima due to excited-state absorption have been determined for the three species. Four components of the relaxation process are distinguished for H2P: the unresolvably short B → Qy internal conversion is followed by the Qy → Qx process, vibrational relaxation, and thermalization in the Qx state with time constant ≈150 fs, 1.8 ps, and 24.9 ps, respectively. Going from H2P to TOxP2+, two processes are resolved, i.e., B → Q internal conversion and thermal equilibration in the Q state. The B → Q time constant has been determined to be 25 ps. The large difference with r...

Time- and polarization-resolved optical spectroscopy of colloidal CdSe nanocrystal quantum dots in high magnetic fields.

Abstract: In an effort to elucidate the spin (rather than charge) degrees of freedom in colloidal semiconductor nanocrystal quantum dots, we report on a series of static and time-resolved photoluminescence measurements of colloidal CdSe quantum dots in ultrahigh magnetic fields up to 45 T. At low temperatures (1.5-40 K), the steady-state photoluminescence (PL) develops a high degree of circular polarization with applied magnetic field, indicating the presence of spin-polarized excitons. Time-resolved PL studies reveal a marked decrease in radiative exciton lifetime with increasing magnetic field and temperature. Except for an initial burst of unpolarized PL immediately following photoexcitation, high-field time-resolved PL measurements reveal a constant degree of circular polarization throughout the entire exciton lifetime, even in the presence of pronounced exciton transfer via Forster energy transfer processes.