B. Di Bartolo

LUMINESCENCE OF CR3+ AND ENERGY TRANSFER BETWEEN CR3+ AND ND3+ IONS IN YTTRIUM ALUMINUM GARNET

The luminescence properties of Cr3+ ions and the energy transfer between Cr3+ and Nd3+ ions have been studied quantitatively in yttrium aluminum garnet (YAG). The energy level diagram and the emission spectrum of 4T2→4A2 transition of Cr3+ ions were calculated in this crystal using the single‐configurational‐coordination model and the absorption data. Our calculated Δ value (the energy difference between the 2E zero‐vibrational level and the crossover point of 2E and 4T2 states), ∼730 cm−1 at room temperature, in contrast to the previously estimated value of ∼1000 cm−1, accounts for a variety of experimental results more satisfactorily. Both the 4T2 and 2E states of Cr3+ ions were found to contribute to the energy transfer to Nd3+ ions. The contribution of the former dominates at high temperatures and the latter at low temperatures. The thermal population distribution between the 4T2 and 2E states of Cr3+ ions was shown to be responsible for the temperature dependences of the Cr3+ luminescence properties ...

Excited state dynamics of thulium ions in Yttrium Aluminum Garnets

Abstract The processes that take place in the excited states of a trivalent Thulium (Tm) ion in an Yttrium Aluminum Garnet (YAG) crystal, being relevant to the use of this system for laser applications, have been the object of several studies. We have reexamined this system, focusing our attention on the dynamics of Tm following its excitation in the 3H4 level. Under these conditions the system relaxes through a cross-relaxation process, 3H4→3F4, 3H6→3F4, whose rate depends upon both the concentration of the Tm ion and the temperature of the crystal. The excitation spectrum obtained by monitoring the 1.8 μm emission of Tm (due to the 3F4→3H6 transition) indicates an increase in the contribution to this emission from the 3H4 level relative to the 3H5 level as the Tm concentration increases; this shows the increased role played by the 3H4 level in pumping the infrared emission. Correspondingly the duration of the luminescence originating in the 3H4 level is shortened as the concentration of Tm increases. The concentration quenching of this lifetime can be fit to a model which assumes that the cross-relaxation is due to a dipole-dipole interaction; from this fit the intrinsic Tm lifetime in absence of cross relaxation can be derived. We have used this lifetime to calculate the rate of the cross-relaxation process. We have evaluated this rate as function of temperature and found it to be fastest at 77 K. We have also calculated the microscopic interaction parameters for the cross-relaxation process by using two independent experimental features: (i) the time evolution of the emission from the 3H4 level, and (ii) the spectral overlap between the 3H4→3F4 emission and the 3H6→3F4 absorption. We have also considered the migration of excitation among the Tm ions in the 3F4 level and calculated the relevant microparameter by the use of the relevant spectral overlap. The data are consistent with the model in which the Tm ions, once excited into the 3H4 level decay by cross relaxation to the 3F4 level, and then transfer rapidly their energy to other Tm ions.

Temperature dependence of the vibronic spectrum and fluorescence lifetime of YAG:Cr3+

The authors have investigated the vibronic spectrum associated with the 2E to 4A2 transition of YAG:Cr3+ in the temperature region 10-300 K. At low temperatures vibrational satellites corresponding to spontaneous phonon emission are observed only on the low energy side of the nophonon R lines which are located at 6863.2 and 6872.6 AA. At higher temperatures, vibronic emission appears also on the high energy side, corresponding to phonon absorption. With increasing temperature, the vibronic bands become more intense relative to the R lines, the peaks broaden, and a fluorescence continuum extends from 6500 to 8000 AA. The fluorescence lifetime of the 2E level which is 8.5 ms at 77 K decreases rapidly above 150 K at 1.6 ms at 300 K. The enhancement of the transition probability of the vibronic emission can account for the major temperature dependence of the fluorescence lifetime without requiring the contribution of purely nonradiative transitions. For this case a calculation of the purely radiative lifetimes can be made. The values are found to be tau rad(R1) approximately 0.07 s and tau rad(R2) approximately 0.09 s.

Downconversion and upconversion dynamics in Pr-doped Y3Al5O12 crystals

We have investigated the spectroscopic properties of praseodymium ion in a Y3Al5O12 sample. We obtained the luminescence spectra by exciting selectively the sample at 460 and 599nm, resulting in the excitation of the P03 and D21 levels, respectively. With excitation in the P03 level we obtained emission from this level and from the D21 level. We used time resolved spectra and decay pattern inspection to assign spectral lines to specific radiative transitions. With excitation in the D21 level we observed luminescence emitted by the same level and upconverted emission from the P03 level and the (4f5d) band. The experimental data indicate that the mechanism responsible for the upconversion processes is excited state absorption. We found the upconverted emission from P03 and the (4f5d) band to be proportional to the square of the excitation energy and to the cube of the excitation energy, respectively.

Radiative and radiationless processes of Er3+ in MnF2

Abstract We have designed a method for the measurement of radiative and radiationless transition rates of fluorescent systems with several metastable levels. This method applies to systems with adjacent metastable levels connected by multiphonon relaxation processes. The measurements required are those of the integrated fluorescence intensities and decay times at different temperatures; no absorption or excitation measurements are required. We have applied this method to the study of the fluorescence of Er3+ in MnF2. The knowledge of both radiative and non- radiative decays has allowed us to reconstruct the kinetics of the excitation and de-excitation processes and to find the dependence on temperature and energy gap of multiphonon relaxation processes in this system.

Optical Spectra and Laser Action of Neodymium in a Crystal Ba2MgGe2O7

Absorption, fluorescence, and excitation spectra of a single crystal of Ba2MgGe2O7: Nd3+ were investigated. The lifetime of the 4F3/2 state of the neodymium ion is 450 μsec at 300°K. The width of the strongest fluorescence line at 1.054 μ is 35 and 10 A at 300° and 80°K, respectively. Laser action was achieved at 1.05444 μ at room temperature.

Evolution of Cl−2 in aqueous NaCl solutions

Following an intense excitation by a photolysing flash, a transient absorption band centered at about 3400 A and approximately 1000 A wide was observed in aqueous NaCl solutions ranging in concentration from 5×10−4 to 2 M. This band was attributed to the presence of Cl−2 ions. The disappearance of Cl−2 was found to be a second process with a rate constant which was dependent on the NaCl concentration. At ambient temperature the kinetics was satisfactorily explained as being due to three simultaneous bimolecular reactions the rate constants of which were obtained. Furthermore, the decay of Cl−2 was observed to follow the same mechanism at elevated temperatures. All three bimolecular reactions were found to have Arrhenius-type temperature dependences.

Applications of the Judd–Ofelt theory to the praseodymium ion in laser solids

Abstract The praseodymium ion in its trivalent state is a very good representative of the entire class of rare earth ions and of the complex spectroscopic properties that they exhibit when placed in solid host lattices. The plethora of its radiative transitions, on one hand, demands the intervention of a theory that could explain their absolute and relative intensities, and, on the other hand, provides a good opportunity for the verification of the validity and of the limits of such a theory. We present our data on the spectral characteristics of trivalent praseodymium in a crystal of barium yttrium fluoride and apply the Judd–Ofelt theory to this system. We then examine the problems presented by the application of the Judd–Ofelt theory to the praseodymium ion and the remedies that have been proposed to ameliorate its performance in the somewhat extreme case provided by the spectra of this ion in solids.

Spectroscopic properties of trivalent praseodymium in barium yttrium fluoride

Abstract We have conducted a spectroscopic investigation of Pr 3+ in barium yttrium fluoride (BaY 2 O 8 ). Two doping concentrations were used: BaY 2 F 8 :Pr 3+ (0.3%) and BaY 2 F 8 :Pr 3+ (1%). The measurements included absorption, luminescence under continuous and pulsed excitations, and thermal effects on some sharp lines. The experimental results were used to characterize this system.

Effects of temperature and concentration on the energy transfer process between erbium and holmium in yttrium aluminum garnet

Abstract We have studied the energy transfer process between Er 3+ and Ho 3+ in YAG by measuring the spectral distribution and the decay patterns of the infrared fluorescence of three samples: YAG: Er (2%), Ho (0.2%), YAG: Er (50%), Ho (2%) and YAG: Er (50%), Tm (6.7%), Ho (1.7%) in a wide range of temperatures 77–700°K. In the first sample the energy transfer process is found to be more efficient at low temperature as shown by the temperature dependence of the Er and Ho fluorescence intensity, the Ho excitation spectrum and the Er and Ho decay patterns. The decay pattern of Er is purely exponential at all temperatures with a lifetime which goes from 3.6 msec at 130°K to 7.6 msec at 575°K. The decay pattern of the Ho fluorescence shows a rise, followed by an exponential decay. In the two other samples, the energy transfer process is very efficient on account of the high Er concentration, as shown by the disappearance of the Er fluorescence, the great enhancement of the Ho fluorescence and the purely exponential decay of the Ho fluorescence. The efficiency of the energy transfer process is so great at all temperatures as to make any variation of it due to temperature unnoticeable. Correspondingly, the Ho fluorescence intensity has the same temperature dependence as the lifetime. The above results are explained in terms of a cross-relaxation model involving the Er and Ho metastable levels.