High-resolution Excitation Spectroscopy Research Articles

Two optical routes of cold carrier injection in silicon revealed by time-resolved excitation spectroscopy

We reveal two routes of optical carrier injection in pure silicon by means of high-resolution excitation spectroscopy on nanosecond cyclotron resonances. Free carriers are generated either by the band-to-band transition assisted by phonon emission, or via two-body collisions of excitons. The first route was previously masked by a strong excitonic response in steady-state optical spectra at low temperatures. Furthermore, valley polarization is achieved for the cold carriers created by the second route with optimized excess energy. These optical carrier injection routes are crucial to initialize the momentum and valley degrees of freedom of carriers in order to enable versatile applications of indirect-bandgap semiconductors.

Sub-Doppler high-resolution excitation spectroscopy of the S1 ← S0 transition of dibenzofuran

Rotationally resolved ultrahigh-resolution fluorescence excitation spectra of the S 1 ← S 0 transition of dibenzofuran have been observed using the technique of crossing a collimated molecular beam and the single-mode UV laser beam. 3291 rotational lines of the 0 0 0 band and 3047 rotational lines of the 0 0 0 + 443 cm − 1 ( 55 0 1 : b 2 ) band have been assigned. The 0 0 0 band has been found to be a b-type transition, in which the transition moment is along the twofold symmetry axis of this molecule, and only the Δ K a = ± 1 transitions were observed. The excited state is identified to be the S 1 1 A 1(ππ ∗) state. In contrast with this, the 0 0 0 + 443 cm − 1 ( 55 0 1 : b 2 ) band has been found to be an a-type transition in which the transition moment is along the long axis in plane. It indicates that the intensity of this vibronic band arises from vibronic coupling with the S 2 1 B 2(ππ ∗) state. We determined the accurate rotational constants and the molecule have been shown to be planar both in the ground and excited states.

Sub-micron scale spectroscopy localized on individual fibers of molecular J-aggregates of pseudoisocyanine dyes in thin polymer film

Local optical properties of molecular J-aggregates on sub-micrometer scale are studied by means of low-temperature high-resolution excitation spectroscopy and by room temperature reflectance microscopy and spectroscopy. Differences in local properties are demonstrated as distributions of absorption line peak positions and line widths

Sub-micron scale spectroscopy and microscopy of individual mesoscopic systems at cryogenic temperatures

We report on a design of a laser scanning solid immersion microscope based on gradient index optics that is suitable for low-temperature excitation spectroscopy on the single molecule level. Further, we present a hole-burning study of a system consisting of molecular J-aggregates and associated trap states. These two studies are pre-requisites for microscopic high-resolution excitation spectroscopy performed on individual fibers of J-aggregates of pseudoisocyanine in a polymer film. Preliminary results of the microscopic study are further compared with microscopic reflection spectroscopy measured on identical systems at room temperature.

Magneto-optics of Ni-bound shallow states in ZnS and CdS.

Transition metals in semiconductors give rise to shallow states which cannot be described on the basis of their 3d wave functions. In this paper a comparative study of shallow states associated with Ni in ZnS and CdS is presented. By means of high-resolution excitation spectroscopy under the influence of magnetic fields the determination of the electronic origin of the observed fine structure becomes possible. The results give insight into the local binding properties as well as the important role of the exchange interaction. Excitation measurements of the $^{3}$${\mathit{T}}_{1}$(P)${\mathrm{\ensuremath{-}}}^{3}$${\mathit{T}}_{1}$(F) ${\mathrm{Ni}}^{2+}$ luminescence reveal weak lines on the low-energy onset of the ${\mathrm{Ni}}^{2+/+}$ charge-transfer band due to the formation of shallow states. The high sensitivity of these measurements allows Zeeman investigations of these nonluminescent shallow states. In both ZnS and CdS all lines exhibit a diamagnetic shift towards higher energies, demonstrating the effective-mass-like nature of the excited states. On the other hand, the fine structure as well as the linear Zeeman behavior is quite different in both host materials indicating the formation of different shallow complexes. In ZnS a transient shallow acceptor state (${\mathrm{Ni}}^{+}$,h) is formed. The zero-phonon-line (ZPL) doublet around 2.437 eV with a zero-field splitting of 1.4 meV develops into a singlet and a triplet (g=0.50) in a magnetic field.It is unambiguously explained by the (${\mathrm{Ni}}^{+}$,${\mathit{h}}_{\mathit{n}=1}$) ground state, taking into account the exchange interaction. An additional ZPL shifted 25.3 meV towards higher energies is at- tributed to an excited (${\mathrm{Ni}}^{+}$,${\mathit{h}}_{\mathit{n}=2}$) state. From the excited hole state as well as from an observed hole-transfer process between ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Cu}}^{2+}$ centers the binding energy of the (${\mathrm{Ni}}^{+}$,h) complex and the ${\mathrm{Ni}}^{2+/+}$ charge-transfer energy are determined to be 108 meV and 2.545 eV, respectively. In CdS a deeply bound electron-hole pair (${\mathrm{Ni}}^{2+}$,e,h) is formed. The ZPL at 2.1904 eV exhibits a zero-field splitting of 50 \ensuremath{\mu}eV attributed to the trigonal crystal field and an isotope shift of -37 \ensuremath{\mu}eV/nucleon. In a magnetic field it splits into a triplet (g=2.26). The Ni-associated shallow complexes in ZnS and CdS differ in the localization of the electron which depends on the deep ${\mathrm{Ni}}^{2+/+}$ acceptor level. In ZnS this level is deep in the band gap; in CdS it is close to the conduction band.

Electronic and vibronic states of the acceptor-bound-exciton complex (A0,X) in CdS. II. Determination of the fine structure of the (A0,XB) electronic states by high-resolution excitation spectroscopy.

In a previous paper [R. Baumert, I. Broser, J. Gutowski, and A. Hoffman, Phys. Rev. B 27, 6263 (1983)] it has been shown that high-density, high-resolution excitation spectroscopy gives new information on the electronic and vibronic excited states of the acceptor--bound-exciton complex (${A}^{0}$,${X}_{A}$) with two holes from the A valence band in CdS. We now report on corresponding results for the (${A}^{0}$,${X}_{B}$) configuration which includes one hole from the second B valence band. This complex is unstable for a very fast B\ensuremath{\rightarrow}A hole conversion, and therefore gives rise to a set of excitation resonances of the ${I}_{1}$ luminescence arising from the (${A}^{0}$,${X}_{A}$) recombination. A detailed theoretical analysis of the energetic structure of the (${A}^{0}$,${X}_{B}$) complex including the dependence on the excitation intensity and on an applied magnetic field allows the correct assignment of the excitation resonances to the (${A}^{0}$,${X}_{B}$) fine-structure levels originating from the interparticle-exchange interactions. It is shown that the magnetic field is a suitable means of distinguishing the different (${A}^{0}$,${X}_{B}$) ground-state levels. The magnetic field also creates allowed transitions which are dipole forbidden in the zero-field case. A self-contained model of the (${A}^{0}$,${X}_{B}$) complex thus can be developed, including all symmetry states and yielding adequate values for the exchange energies within the complex.

High-resolution measurements of the triplet t 1 davydov components in naphthalene N- h8 AND N- d8

Absorption lines of the first singlet-triplet 0,0 transition in naphthalene crystals are investigated at 1.6 K using high-resolution excitation spectroscopy with cw dye lasers. The observed substructure of the upper Davydov component is explained quantitatively both for N- h 8 and N- d 8. It originates from impurity Icvels of isotopes in natural abundance, which are not amalgamated in the triplet exciton band.

High resolution excitation spectroscopy on triplet excitons in organic molecular crystals comparison of naphthalene and anthracene

Triplet exciton absorption spectra of naphthalene and anthracene have been measured indirectly applying phosphorescence and delayed fluorescence excitation spectroscopy in the temperature range between 2 K and 300 K. The temperature dependence of the exciton line shape and line width is very similar in both crystals indicating comparable exciton phonon coupling. The line positions however, are shifted in different directions between helium and room temperature, to the red for naphthalene and to the blue for anthracene. This is explained with the thermal expansion of the crystal affecting the solvent shift with opposite sign in these crystals. In the naphthalene spectra below 30 K a substructure in the upper 0.0 Davydov component is observed, which can not be explained so far.