Multi-phonon relaxation of the H− local mode in CaF2

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ELSEVIER

JOURNAL

LUMINESCENCE Journal of Luminescence 76&77 (1998) 628-63 I

Multi-phonon C.P. Davison”,

OF

relaxation of the H- local mode in CaF2

J.A. Campbellb,

J.R. Engholm”,

H.A. Schwettman”,

U. Happek”.*

Abstract We have investigated the non-radiative relaxation of the substitutional H local mode (18= 965 cm- ‘) in CaFZ. This system is attractive due to its simplicity. the only possible decay mechanism. aside from radiative relaxation, is non-radiative multiphonon decay into host phonons. Saturation experiments were performed at the Stanford Picosecond Free Electron Laser Center. using a pump-probe technique. We find an energy relaxation time T, of 45 ps at low temperature. We compare this result with the relaxation of the local mode of an interstitial H ion next to a Lanthanum impurity ion. and find a much more rapid relaxation time T, of 6 ps. (: 1998 Elsevier Science B.V. All rights reserved. PACT: 63.2O.P~; 41.60.Cr: 78.50.E~ Ke~~n~~u’s: Local modes: Free electron

laser; Non-radiative

relaxation;

The spectroscopic studies of H- local modes in ionic crystals date back to 1960, when Schafer [l] reported the observation of the U-center local mode in KCI. While a large number of papers have been published since [2], there are only a few reports about non-linear measurements to study the relaxation of the excited H- local mode. However, two groups have investigated the relaxation of the H- local modes in CaF2. In 1971 Lee and Faust [3] observed the saturation of the local mode of substitutional hydrogen ions (H,) in CaFz using a TEA CO1 laser [3]. More recently, Lang et al. [4] reported a relaxation time of 17 ps at low temperatures for the same mode, using ns IR pulses of a tunable high-pressure CO, laser.

*Corresponding 00X-23 PII

I3/98/$19.00 SOO22-23

author. Fax: + 1 706 542 2492 I’ 13(97)00

I998

Elsevier Science B.V. All rights reserved 167- 1

CaF,

The substitutional H; ion replaces a regular F- ion in the CaF, lattice (Fig. l), and a single vibrational line occurs at 965 cm- ’ at low temperature, with a width of about 0.2cm-‘. Interstitial Hip ions can be introduced into CaF, crystals doped with trivalent rare-earth ions ( RE3 ‘), where the hydrogen ion replaces a charge compensating interstitial fluorine ion (Fig. 1). The vibrational spectrum consists of two lines, where the higherfrequency 11’corresponds to a vibration along the RE3+-H; axis, and a lower-energy doublet at frequency P associated with vibrations perpendicular to the RE3+-H; axis. The center-of-gravity of these lines lies at about 1070cm- ‘, while the splitting depends on the RE3+ ion and ranges from about 30cm-’ for Lu3+ to 145cm-’ for La3+[5]. While the non-radiative relaxation of the H,excited local mode can occur in a multiphonon

C.P. Darison

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Z

1 I

I

I I

H-

Fig. I. The fluorite tional IH,

) hydrogen

lattice with interstitial

(H, )

and substitu-

s

Fig. 2. Energy level diagram

X-Y

i

H;

for H,

and H,

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decay process into host phonons, the high-frequency mode \I’ of the Hi- mode has an additional decay channel into the v~)‘mode (Fig. 2). An indication of this decay channel is the larger line width of the high-frequency mode 11’ compared to rXY, although direct experimental results on the relaxation time of the interstitial local mode are lacking. In this paper we present results of the relaxation time of both substitutional (H,) and interstitial (H;) local modes in CaF2 using a ps infrared freeelectron laser (FEL). The samples of pure CaFz and CaF,:0.5%La3+ were grown in an RF-furnace at the University of Canterbury and hydrogenated by heating the crystals in contact with molten aluminum to 850°C in a hydrogen atmosphere for several hours [6]. The finished samples were characterized by linear spectroscopy using a Bruker IFS66v FTIR spectrometer. equipped with a temperature-variable cryostat. The relaxation measurements were performed at the Stanford Picosecond FEL Center. The Stanford FEL recently extended its operating range from 1000 to 850 cm-‘, allowing one to measure both the substitutional local mode at v, = 965 cm- ’ and the interstitial local mode associated with the Lanthanum ion at V’ = 1122 cm-‘. The relaxation rates are determined by a picosecond pump-probe saturation technique, wherein a weak probe pulse of ps duration, resonant with the transition of the

H- local mode, probes the bleaching of the absorption caused by a strong pump pulse at the same frequency. Simultaneous resonant excitation of higher transitions is disallowed by the anharmonicity of the local mode. The output of the FEL laser consists of macropulses with a duration of 3 ms and a repetition rate of 20 Hz. Within a macropulse, a train 01 picosecond-long pulses are emitted with a repetition rate of 11.8 MHz. To avoid excessive heating of the sample due to the high average intensity of the strongly focussed beam, about 140 micropulses are selected per macropulse using an acousto-optic deflector. During the experiments, the center frequency and both spectral and temporal widths of the pulses are monitored. The selected pulses are split into a pump and a probe pulse. These are focussed with an off-axis parabolic mirror onto the sample, which is mounted onto the cold finger of a temperature-variable cryostat using indium foil as a thermal link. The micropulse characteristics of the laser output are, within limits, adjustable. In our experiments the spectral width of the laser pulses was 5 cm ‘, the temporal width 3 ps, and the micropulse energy at the sample about 40 nJ. Fig. 3 shows a typical transmission curve for the H, local mode at low temperature (8 K) as a function of the delay time between the pump and the probe pulses. In the small saturation limit,

C. P. Davison

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4.8, ,

g

4.6

5 r;

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E

4.0

. 5

l0

50

Delay

Fig. 3. Pump-probe between pump and T = 10 K.

150

100

200

(PS)

transmissmn signal as a function of delay probe pulses for the H;~ local mode at

the relation between the probe signal Z(At) and the delay time At between pump and probe pulses can be written as Z(At) z exp( - fdt), where Z is the relaxation rate of the excited state. Note that no parameters such as the absorption coefficient of the sample, line width of the laser, etc., are needed to interpret the saturation data in a pump-probe experiment. The solid line in Fig. 3 represents a single exponential fit to the data with a f corresponding to a relaxation time Ti of 45 & 3 ps. This value is a factor of three larger than that published by Lang et al. [4] using ns pulses to saturate the local mode transition. In the saturation technique used by Lang et al., the transmission is monitored as a function of the laser intensity I, and the absorption coefficient r decreases with I according to the relation x = z,(l + Z/Z&‘, where Is is the saturation intensity. The relaxation time Ti can then be calculated using Ti = (htti)/(ZoZs). In this equation, g is the absorption cross section, which is not well known for the HP local mode, thus an incorrect value of T1 may result. In addition, we observe a power broadening of the H- absorption line during the interaction with the ps FEL pulse. While this causes no problem in the interpretation of the pump-probe transmission signal aside from artifacts around t = 0, power broadening should be included in the interpretation of incoherent saturation data using pulse lengths longer than the relaxation time.

+

3.811

0

I

I

10

20 Delay

I

30

(PS)

Fig. 4. Pump -probe transmission signal as a function of delay between pump and probe pulses for the Hi local mode at T = IO K.

We also succeeded in saturating the r’ = 1122 cm-’ absorption line of the Hi- interstitial mode in a La3+-doped CaF, crystal. Fig. 4 shows the pump-probe transmission signal. This local mode relaxes in Ti = 6 + 2 ps, much more rapidly than the substitutional local mode. Although the H, and Hi- local modes are not equivalent, we attribute the more rapid relaxation of the interstitial mode to the additional relaxation channel into the low-frequency I!-‘~modes in a one-phonon process. Experiments are under way to saturate the rXYabsorption line of the interstitial mode. In conclusion, we have saturated both the substitutional and interstitial H- local modes in CaF, using a ps pump-probe technique. We find a relaxation time of 45 ps for the substitutional mode, a factor of three longer than previously reported. For the first time, we measured the relaxation time of the interstitial H- local mode and attribute the rapid relaxation time of 6 ps to a one-phonon decay into a local mode of lower energy. This work was supported by the Office of Naval Research, Grant No. NOOO14-94-1-1024. U.H. likes to acknowledge the University of Canterbury in Christchurch, New Zealand, for support and hospitality during a Visiting Erskine Fellowship and G.D. Jones for stimulating discussions.

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