CaBa-铝酸盐玻璃中Cr4+谱线宽度随温度的变化

测量了15~300K温度范围内57.5%Al_1.5O-35%CaO-7.5%BaO玻璃中四价铬的发射光谱.这种材料中铬离子的能级处于Tanabe-Sugano图上弱场范围中,最低的激发态是³T₂,发射谱是一个宽带.按照单频近似理论拟合低温下的光谱,得到³T₂能级的零声子线位置E_zp=8400cm^-1,声子能量tω=320cm^-1,黄昆因子S=358.尽管单频近似能够较好地描述低温下的线形,发射光谱宽度随温度的变化却与单频近似理论的结果不符.讨论了这种差别的原因,认为可能的解释是与激发态耦合的声子能量大于与基态耦合的声子能量.

Temperature Dependence of the Emission Band Width of Cr4+ in CaBa-Aluminate Glass

The emission spectra of Cr⁴⁺ in 57.5%AlO_1.5-35%CaO-7.5%BaO glass were measured at different temperature from 15K to 300K. The energy levels of Cr⁴⁺ in this host fall into the weak field regime on the Tanabe Sugano diagram, the lowest excited state is ³T₂, and the emission spectrum is a broad band (Fig.2). The spectrum at 15K (Fig. 1) can be approximated by a Pekarian^5] g(E_zp-ptω)=e^-SS^P/p! By fitting the spectrum with this function we have got E_zp=8400±13cm^-1, ω=320±5cm^-1, S= 3.58±0.08. Strictly speaking, the spectrum should be the convolution of the phonon sideband and an inhomogeneous lineshape, which is usually expressed by a Gaussian. By calculating the convolution and comparing with Fig.1, we estimated that the inhomogeneous line width is within 300~400cm^-1 (FWHM). The emission band widths obtained from Fig.2 are shown in Fig.3 as solid cycles. According to the single frequency approximation, its temperature dependence would be^5] D=D₀1+e^-tω/kT/1-e^-tω/kT]^1/2=D₀coth^1/2tω/kT], where D₀ is the band width at 0K. With the parameters obtained above, D should be as shown by the solid line in Fig.3 (labeled by ωV/ωU=1). Evidently, it does not agree with the experimental result. The temperature dependence of the width of the phonon sideband originates from the thermal excitation of higher vibronic states. Transition from these states covers larger energy range. If the high vibrational states coupled to the excited electronic state are less populated, the temperature dependence of the bandshape would be weaker. This will happen if the phonon coupled to the excited electronic state possesses energy larger than that coupled to the ground state. Thus it makes us apply the model where the ground state and the excited state couple unequal frequency phonons^6]. Keeping S, E_zp and ηω_U obtained from the single frequency approximation fitting and taking ω_V/ω_U as a new parameter, we calculated the temperature dependence of the emission bandwidths. The calculated results are shown in Fig.3, ω_V/ω_U=1.15 results in the best fitting.