Excited State Absorption Spectra Research Articles

Excited-state absorption of Co2+ in MgF2 and KZnF3

We report on measurements of excited-state absorption spectra and absolute cross-sections in Co2+-doped KZnF3 and MgF2 at room temperature in the near-infrared domain. The data are compared with the predictions of the single-configuration co-ordinate model.

Energy-level assignments for the 1E and 3T1a states of MgO:Ni2+

The $^{1}$${\mathit{T}}_{2}$${\ensuremath{\rightarrow}}^{1}$E${,}^{3}$${\mathit{T}}_{1}$a luminescence, and the $^{3}$${\mathit{T}}_{2}$${\ensuremath{\rightarrow}}^{1}$E${,}^{3}$${\mathit{T}}_{1}$a excited-state absorption spectra of MgO:${\mathrm{Ni}}^{2+}$ have been observed for the first time. These data are used in conjunction with the conventional ground-state absorption spectrum, to determine the energy-level assignments for the $^{3}$${\mathit{T}}_{1}$a and $^{1}$E states. In particular, three of the four spin-orbit components of the $^{3}$${\mathit{T}}_{1}$a state are identified. Although the ${\mathit{A}}_{1}$ spin-orbit component is broadened beyond recognition, the ${\mathit{T}}_{1}$ component is directly observed as a magnetic-dipole allowed transition in the $^{3}$${\mathit{T}}_{2}$${\ensuremath{\rightarrow}}^{3}$${\mathit{T}}_{1}$a excited-state absorption spectrum. Lastly, the energies of the ${\mathit{T}}_{2}$ and E spin-orbit levels are inferred from the positions of their vibrational sidebands in the ground-state absorption spectrum.

Amplification in the 1300 nm telecommunications window in a Nd-doped fluoride fibre

Gain values of 4 to 5 dB were observed between 1310 and 1370 nm in a Nd-doped bimoded ZBLANP fibre pumped at the diode laser compatible wavelength of 795 nm. The magnitude of the gain was shown to be limited by saturation due to amplified spontaneous emission from 1050 nm radiation. The excited state absorption spectrum was mapped.

Synthesis and study of a mixed-ligand ruthenium(II) complex in its ground and excited states: bis(2,2′-bipyridine)(dipyrido[3,2-a : 2′,3′-c]phenazine-N4N5)ruthenium(II)

The ruthenium (II) complex [Ru(bipy)2(dppz)]2+(bipy = 2,2′-bipyridine, dppz = dipyrido[3,2-a : 2′,3′-c]phenazine) was synthesized, characterized, and studied. Its oxidation and first reduction potentials are respectively 1.24 and –1.02 V (vs. saturated calomel electrode). The maxima of the metal-to-ligand charge-transfer absorption and emission occur respectively at 448 and 610 nm. These data suggest that [Ru(bipy)2(dppz)]2+ is made up of two electronically independent units, one behaving as a [Ru(bipy)3]2+-like chromophore, the other as a phenazine-like electron acceptor. Excited-state absorption spectra were obtained for [Ru(bipy)2(dppz)]2+ and its parent complex [Ru(bipy)3]2+. Above 500 nm the latter shows only one maximum (at 510 nm), and the former shows two maxima (around 526 and 557 nm), whereas radical anions of both bipyridine and phenazine or dipyridophenazine show two maxima (at 512 and 552 nm for phenazine). In the case of [Ru(bipy)2(dppz)]2+ these results can be interpreted in terms of a light-induced directed charge transfer from the ruthenium to the phenazine part of the dipyridophenazine ligand, and its localization on this ligand moiety. Photochemical properties of [Ru(bipy)2(dppz)]2+ were studied in ethanol. The excited state of the complex is quenched both by an electron acceptor (methylviologen, kq= 1.37 × 109 dm3 mol–1 s–1) and an electron donor (triethanolamine, kq= 4.40 × 107 dm3 mol–1s–1).

Optical-absorption and photoionization measurements from the excited states of Ce3+:Y3Al5O12.

The optical transitions in ${\mathrm{Ce}}^{3+}$:${\mathrm{Y}}_{3}$${\mathrm{Al}}_{5}$${\mathrm{O}}_{12}$ from the lowest 5d excited state of the ${\mathrm{Ce}}^{3+}$ ion to the host conduction band have been investigated. Two spatially overlapped and time-sequenced laser pulses were used to observe directly the excited-state absorption (ESA) and the two-step photoionization of the cerium ions. The peak cross section of the ESA transition is (1.0\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}17}$ ${\mathrm{cm}}^{2}$ at 700 nm. The ESA spectrum was measured from 600 to 880 nm and is used to construct an energy-level diagram for the ${\mathrm{Ce}}^{3+}$ ions relative to the energy-band states of the ${\mathrm{Y}}_{3}$${\mathrm{Al}}_{5}$${\mathrm{O}}_{12}$ host. The energy gap between the lowest relaxed 5d state and the conduction-band edge is estimated to be 10 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Strong-probe saturation measurements indicate a slow and inefficient back-transfer of excitation to the cerium ions following the ESA. Photoconductivity measurements were used to confirm the two-step photoionization of the cerium ions.

Excited-state absorption of Cr3+ in LiCaAlF6: Effects of asymmetric distortions and intensity selection rules.

We have measured the \ensuremath{\pi}-polarized excited-state absorption (ESA) spectra of the new laser material, ${\mathrm{LiCaAlF}}_{6}$:${\mathrm{Cr}}^{3+}$ (hereafter, ${\mathrm{Cr}}^{3+}$:LiCAF). We have found that the peak cross section of the ESA band is 0.17\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}20}$ ${\mathrm{cm}}^{2}$. We are therefore able to explain the high efficiency previously observed for the ${\mathrm{Cr}}^{3+}$:LiCAF laser by noting that the peak emission cross section of 1.3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}20}$ ${\mathrm{cm}}^{2}$ is much larger than this ESA cross section. As a result, the ESA does not diminish the value of the effective stimulated-emission cross section, in spite of its tendency to overlap the emission band in most ${\mathrm{Cr}}^{3+}$-doped crystals. It is then shown that the ESA transition is weak relative to the emission band because the ${t}_{2u}$ distortion present at the ${\mathrm{Al}}^{3+}$ site where the ${\mathrm{Cr}}^{3+}$ substitutes adds transition strength to the \ensuremath{\pi}-polarized $^{4}\mathrm{A}_{2}$${\mathrm{\ensuremath{-}}}^{4}$${\mathrm{T}}_{2}$ absorption and emission features, while it does not contribute oscillator strength to the ESA band. The $^{4}\mathrm{T}_{2}$${\ensuremath{\rightarrow}}^{4}$${\mathrm{T}}_{1}$a ESA band is found to peak near 10 030 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, rather than near 7600 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ as expected on the basis of simple crystal-field theory. This is explained as being due to the influence of non-totally-symmetric distortions in the excited states of ${\mathrm{Cr}}^{3+}$.

Excited-state absorption by Eu2+ in KBr, KCl, and NaCl.

Excited-state-absorption (ESA) measurement have been made on alkali halide crystals doped with ${\mathrm{Eu}}^{2+}$, including both as-received and thermally quenched samples. No ESA has been detected in the visible spectral region, but strong excited-state absorption occurs in the near ultraviolet. This is in qualitative agreement with the published excited-state ionization model of ESA in ${\mathrm{CaF}}_{2}$:${\mathrm{Eu}}^{2+}$. However, the observed ESA onset energies in alkali halides are significantly lower than predicted by that model, and are more nearly consistent with a charge-transfer transition. The ESA spectra show little evidence of ground-state depletion, and together with data on the fluence dependence of fluorescence and laser transmission, this suggests that the quantum efficiency of ${\mathrm{Eu}}^{2+}$ in these hosts is rather low.

Direct observation of the excited-state c i s–t r a n s photoisomerization of bacteriorhodopsin: Multilevel line shape theory for femtosecond dynamic hole burning and its application

A dynamic hole-burning study of light-adapted bacteriorhodopsin (BR568) using 6 fs optical pulses has recently been reported [R. A. Mathies, C. H. Brito Cruz, W. T. Pollard, and C. V. Shank, Science 240, 777 (1988)]. The temporal evolution of the excited state absorption and emission spectra after excitation with 60 fs pulses provides a direct observation of the C13=C14 torsional isomerization of the retinal chromophore on the excited state potential surface. Here, we present a more detailed discussion of these spectra. The transient hole line shapes are then calculated by solving the density matrix equations for the third-order susceptibility of a multilevel system. The resulting equations are written without reference to the individual vibronic transitions by using the absorption correlation function 〈i‖i(t)〉. The calculations show that the sharp features seen at short delays arise from coherence coupling effects which occur when the pump and probe pulses overlap in time. This analysis demonstrates that the hole seen at 60 fs is consistent with the broad homogeneous absorption line shape for BR568 originally predicted from resonance Raman intensities, and points out the utility of 〈i‖i(t)〉, derived from resonance Raman intensity analysis, in understanding femtosecond dynamic hole-burning experiments.

Pump excited-state absorption in erbium-doped fibers

Ground-state and excited-state absorption spectra covering the wavelength range of 450-1050 nm are presented for erbium-doped silica fibers with four different core codopants: GeO(2), GeO(2)/B(2)O(3), GeO(2) /P(2)O(5), and Al(2)O(3). It is shown that the host glass influences the excited-state absorption spectra and that P(2)O(5)- or A1(2)O(3)-codoped fibers are the preferred choice for 514.5-, 655-, or 807-nm pump wavelengths owing to reduced pump excited-state absorption. However, excited-state absorption is still significant at the 807-nm wavelength. Pump wavelengths of 524, 532, and 980 nm appear ideal because of the strong ground-state absorption and lack of excited-state absorption.

Photophysical properties and laser performance of rubrene

The potential of rubrene as a laser dye was investigated in deaerated solutions with a XeCl excimer laser as pump source. Laser tuning range, quantum efficiency and photostability as well as excited singlet state absorption (ESA) spectra were measured. The (S1-S2) absorption band strongly overlaps with the fluorescence spectrum and thus ESA constitutes an important loss factor in the lasing process while triplet absorption can be neglected in the wavelength region of interest. Laser performance of rubrene is superior compared to other polycyclic aromatic hydrocarbons which can be considered as model compounds of the lasing process in organic dyes.

Quantum electronic properties of the Na/sub 3/Ga/sub 2/Li/sub 3/F/sub 12/:Cr/sup 3+/ laser

Few of the existing Cr/sup 3+/ vibronic lasers have achieved the slope efficiency and tuning range expected based on their known spectroscopic properties. To discover the cause of this behavior, the performance of chromium-doped gallium fluoride garnet, Na/sub 3/Ga/sub 2/Li/sub 3/F/sub 12/:Cr/sup 3+/, as a laser material has been investigated experimentally. The data reported include absorption and emission spectra, emission rates, quantum efficiency, laser wavelength tuning range, laser output slope efficiencies, and excited-state absorption spectra. Similar properties of the alexandrite laser material were studied for comparison. The results indicate that the performance of the gallium fluoride garnet laser is severely limited by Cr/sup 3+/ excited-state absorption (ESA). A model is presented to account for the unexpected nature of the ESA, which appears to be a common problem for all Cr/sup 3+/ vibronic lasers. Criteria are suggested for choosing Cr/sup 3+/ hosts for which the effects of ESA will be minimized. >

Excited state absorption of Sm2+ in SrF2 and SrCl2

The excited state absorption spectrum of Sm2+ diluted as an impurity in the single crystal hosts SrF2 and SrCl2 has been observed. The absorption peaks associated with the 5D0(4f6) excited state were found to be displaced by approximately 3000 cm−1 from the absorption bands arising from the 7F0(4f6) ground state when the 5D0–7F0 energy was taken into account. The observed peak separation is ascribed to the 4f–5d exchange interaction. From the measured peak positions and band intensities, the exchange energy for the Sm2+ impurity in the two host crystals was determined to be about the same order of magnitude as that expected for the free ion. The implications of the properties of the excited state absorption bands for the performance of CaF2:Sm2+ and SrF2:Sm2+ lasers are considered.

Excited-state absorption spectra of V2+ in KMgF3 and MgF2.

We have measured the excited-state absorption spectra of ${\mathrm{V}}^{2+}$ in ${\mathrm{KMgF}}_{3}$ and ${\mathrm{MgF}}_{2}$. The observed absorption bands can be identified as being due to the $^{4}\mathrm{T}_{2}$\ensuremath{\rightarrow}${\mathrm{}}^{4}$${\mathrm{T}}_{1}$a and $^{4}\mathrm{T}_{1}$b transitions. The position and shape of the $^{4}\mathrm{T}_{2}$\ensuremath{\rightarrow}${\mathrm{}}^{4}$${\mathrm{T}}_{1}$b transition are determined primarily by the displacement in the ${a}_{1g}$ mode while the coupling of the orbital triplet states to ${e}_{g}$ distortions is found to have a substantial influence on the nature of the $^{4}\mathrm{T}_{2}$\ensuremath{\rightarrow}${\mathrm{}}^{4}$${\mathrm{T}}_{1}$a transition. In fact, this transition is observed as a broad band rather than as a sharp line, as is predicted solely on the basis of the ${a}_{1g}$ coordinate. The $^{4}\mathrm{T}_{2}$\ensuremath{\rightarrow}${\mathrm{}}^{4}$${\mathrm{T}}_{1}$a transition is shifted up in energy relative to its predicted position and significantly overlaps the emission spectrum, thereby providing an explanation for the poor lasing efficiency reported for ${\mathrm{V}}^{2+}$ lasers.

Triplet-State Kinetics of 2,5 Diphenyloxazole (PPO)

Time-resolved excited-state triplet-triplet absorption spectra were measured for solutions of 2,5 diphenyloxazole (PPO) in various solvents, with the use of a pump and probe technique. The rate constants for intersystem crossing, triplet deactivation by oxygen, and triplet-triplet self-quenching are obtained. The latter two rate constants are substantially larger than the corresponding rate constants for 2-(1 naphthyl)-5-phenyloxazole-αNPO.

The Regularities in Ground‐ and Excited‐State Absorption Spectra of Impurity Ions of the Iron Group in Crystals

AbstractThe excited‐state absorption spectra (ESS) of crystalline materials doped with ions with dn‐shells are studied theoretically. The method is used of comparison of the ESS with the ground‐state absorption spectra (GSS). Classification of the ESS into three types is made using the electron distribution in the t2 and e subshells of the strong crystal field and the spin values and the regularities for each type are determined. Comparative diagrams of the transition frequencies from the excited states and from the ground states versus Dq/B are drawn. The cases are determined in which the oscillator strengths of the broad band transitions from the excited states can be calculated in the parameters which are determined using the GSS and the oscillator strengths are estimated. The results show that the GSS of the impurity ions with unfilled d‐shells can be used to evaluate the ESS characteristics.

Excited state absorption of V 2+ and Cr 3+ ions in crystal hosts

We have measured the excited state absorption spectra of KMgF 3: V 2+, Na 3Ga 2Li 3F 12:Cr 3+, and BeAl 2O 4:Cr 3+, and BeAl 2O 4:Cr 3+. The 4T 2→ 4T 1a, 4T 1b transitions were observed for the first two systems, while the 2E, 2T 1→ 2T 2 transitions were seen for BeAl 2O 4:Cr 3+. All excited state absorptions, except the 4T 2→ 4T 1 a transition, can be understood on the basis of simple crystal fields theory. The 4T 2 → 4T 1 a transition is found to be profoundly modified by the Jahn-Teller effect, so that it is broader and at higher energy than ohterwise expected. This excited state absorption is observed to overlap the emission spectrum of KMgF 3:V 2+, and thereby substantially reduces the effective stimulated emission cross-section.

Xe 2Cl and Kr 2F excited state (4 2Γ) absorption spectra: measurements of absolute cross sections

Absolute absorption cross sections for the lowest excited states (4 2Γ) of Xe 2Cl and Kr 2F have been measured at discrete wavelengths in the visible and ultraviolet (Xe 2Cl: 325 ⩽ λ ⩽ 495 nm; Kr 2F: 248 ⩽ λ ⩽ 570 nm). The wavelengths observed for the 9 2Γ ← 4 2Γ band peak in each spectrum confirm the predictions of theory. However, the spectral profile and magnitude of the Xe 2Cl and Kr 2F absorption cross sections are considerably different from those predicted theoretically for the corresponding rare gas dimer ions, Xe + 2 and Kr + 2.

Optical properties and formation of oxygen-perturbed F2 + color center in NaCl.

The optical properties of a recently discovered laser-active color center in oxygen-doped NaCl are reported. The color center is produced through a two-step photoaggregation process in additively colored crystals. Optical spectroscopy indicates that the point defect is a perturbed ${F}_{2}$${\mathrm{}}^{+}$ center. The center has very broad absorption and emission bands peaking at 1.09 and 1.55 \ensuremath{\mu}m, respectively, with a dipole moment oriented along the 〈110〉 directions of the crystal. The center population can be fully aligned along any 〈110〉 direction by using two-photon pumping. Ground- and excited-state absorption spectra compare well with the dielectrically embedded ${\mathrm{H}}_{2}$${\mathrm{}}^{+}$ model for the eight transitions observed. uv spectroscopy has identified the substitutional ${\mathrm{O}}^{2\mathrm{\ensuremath{-}}}$ ion as being the necessary impurity for the center formation. The proposed model for the color center is an ${\mathrm{O}}^{2\mathrm{\ensuremath{-}}}$-perturbed ${F}_{2}$${\mathrm{}}^{+}$ defect. The perturbing ${\mathrm{O}}^{2\mathrm{\ensuremath{-}}}$ ion serves both as a stable electron trap as well as a spatial trap for the ${F}_{2}$${\mathrm{}}^{+}$ center.

Influence of magnetic field on the photochemical reactivity of coordination complexes: photophysics and photochemistry of chromium(III) polypyridine and hexacyanocobaltate(III) complexes

Intense magnetic fields, H > 10 kG, induce changes in the absorption and emission spectra of the long-lived doublet excited states in Cr(III) polypyridine complexes and increase the rate of excited-state relaxation by ca. 10%. Similar effects were detected with polycrystalline samples of K/sub 3/(Co(CN)/sub 6/) between -35 and -130/sup 0/C, while a 100% increase of the quantum yield for the cyano aquation was measured in room temperature photolyses under a 24-kG field. The effect of the field on the photophysics and photochemistry of the complexes is discussed in terms of a mechanism that involves the coupling of the field to the excited-state electronic angular momentum and the effect of such a coupling on the transition probabilities.

Excited state absorption of Cr3+ in K2NaScF6 and Gd3Ga2(MO4)3, M=Ga, Al

The excited state absorption (ESA) spectra of Cr3+ in the elpasolite K2NaScF6 and the garnets Gd3Sc2(GaO4)3 (GSGG) and Gd3Sc2(AlO4)3 (GSAG) were measured from 0.3 to 1.8 μ, exclusive of the fluorescence region, using pulsed laser excitation. The two ligand field transitions 4T1g(a)←4T2g and 4T1g(b)←4T2g were located in the garnets at 9000 and 19 400 cm−1 (19 200 cm−1 for the elpasolite). In addition, intense UV charge transfer bands were found in the garnets, but not the elpasolite. The elpasolite spectrum confirms that major differences exist between the ESA of 2Eg and 4T2gCr3+ excited states. A single configuration coordinate model is developed which is adequate for predicting energies but not the bandwidths of 2Eg and 4T2g ESA transitions between ligand field levels.