An investigation was made of the dependences of the initial I(5 2 P 1/2 )→I(5 2 P 3/2 )+ hν ( λ = 1315nm) luminescence signal on the pressure and nature of the buffer gases. Atoms of I in the 5 2 P 1/2 state were formed, in initial concentrations of (0.5–3)×10 16 cm −3 , as a result of photolysis of CF 3 I (0.01 atm) by the fourth harmonic of a Q -switched neodymium laser ( E ∼0.6 J, τ ∼2×10 −3 sec). The I(5 2 P 1/2 ) luminescence was recorded by a germanium photodiode whose time constant was (3–4)×10 −7 sec. When CF 3 I was diluted with Ar, Xe, and SF 6 buffer gases, the intensity of the luminescence of the I(5 2 P 1/2 ) atoms increased because of an increase in the probability of the 5 2 P 1/2 →5 2 P 3/2 transition as a result of collisions of the iodine atoms with the atoms or molecules of the buffer gases. The dependence of the probability of the investigated transition on the nature and pressure of the buffer gases was determined in the pressure range ∼0–1 atm. Estimates were obtained of the oscillator strengths corresponding to the matrix elements of the induced electric dipole moment due to the deformation of the electron shells of the iodine atoms in collisions.
An investigation was made of the dependences of the initial I(5 2 P 1/2 )→I(5 2 P 3/2 )+ hν ( λ = 1315nm) luminescence signal on the pressure and nature of the buffer gases. Atoms of I in the 5 2 P 1/2 state were formed, in initial concentrations of (0.5–3)×10 16 cm −3 , as a result of photolysis of CF 3 I (0.01 atm) by the fourth harmonic of a Q -switched neodymium laser ( E ∼0.6 J, τ ∼2×10 −3 sec). The I(5 2 P 1/2 ) luminescence was recorded by a germanium photodiode whose time constant was (3–4)×10 −7 sec. When CF 3 I was diluted with Ar, Xe, and SF 6 buffer gases, the intensity of the luminescence of the I(5 2 P 1/2 ) atoms increased because of an increase in the probability of the 5 2 P 1/2 →5 2 P 3/2 transition as a result of collisions of the iodine atoms with the atoms or molecules of the buffer gases. The dependence of the probability of the investigated transition on the nature and pressure of the buffer gases was determined in the pressure range ∼0–1 atm. Estimates were obtained of the oscillator strengths corresponding to the matrix elements of the induced electric dipole moment due to the deformation of the electron shells of the iodine atoms in collisions.
An investigation was made of the dependences of the initial I(5 2 P 1/2 )→I(5 2 P 3/2 )+ hν ( λ = 1315nm) luminescence signal on the pressure and nature of the buffer gases. Atoms of I in the 5 2 P 1/2 state were formed, in initial concentrations of (0.5–3) × 10 16 cm −3 , as a result of photolysis of CF 3 I (0.01 atm) by the fourth harmonic of a Q -switched neodymium laser ( E ∼0.6 J, τ ∼2 × 10 −3 sec). The I(5 2 P 1/2 ) luminescence was recorded by a germanium photodiode whose time constant was (3–4) × 10 −7 sec. When CF 3 I was diluted with Ar, Xe, and SF 6 buffer gases, the intensity of the luminescence of the I(5 2 P 1/2 ) atoms increased because of an increase in the probability of the 5 2 P 1/2 →5 2 P 3/2 transition as a result of collisions of the iodine atoms with the atoms or molecules of the buffer gases. The dependence of the probability of the investigated transition on the nature and pressure of the buffer gases was determined in the pressure range ∼0–1 atm. Estimates were obtained of the oscillator strengths corresponding to the matrix elements of the induced electric dipole moment due to the deformation of the electron shells of the iodine atoms in collisions.
The influence of molecular collisions on the production of the degenerate four-wave mixing signal in I(2) is presented. Measurements were performed on gaseous molecular iodine, I(2), contained in a glass cell in which pressure, temperature, and species concentration are easily and independently varied. Frequencydoubled outputs from a seeded Nd:YAG laser and an excimer-pumped dye laser were used as excitation sources. We have studied the dependence of signal strength versus buffer gas pressure, with pump intensity as a third parameter. It is evident from our results that, for pump intensities of less than 1 MW/cm(2), the pressure dependence of the signal follows that given by a simple two-level model in the homogeneously broadened regime. In this regime collisional deexcitation becomes significant, leading to changes in saturation intensity. This is evidenced by a reduction in the signal with an increase in buffer gas pressure. This behavior is similar to that seen in laser-induced fluorescence. At higher pump intensities, the signal is seen to increase with pressure; this behavior cannot be described by the simple two-level model.
It is shown that the stimulated emission cross section σ 3—4 of the 2 P 1/2 ( F =3)→ 2 P 3/2 ( F' =4) transition in iodine can be determined without allowance of the influence of chemical reactions on the population inversion and mixing of sublevels with the ground 2 P 3/2 and excited 2 P 1/2 states of iodine. A report is given of measurements of the σ 3—4 cross section as a function of the pressure of CF 3 I, n -C 3 F 7 I, and i -C 3 F 7 I vapors and of He, Ne, Ar, and Xe inert gases. The collisional (impact) widths of the transition profile and of the coefficients of collisional broadening by these gases are obtained. A comparison is made with the theory and with the results of published investigations.
The disturbance of the optical homogeneity of high-energy, 1.315 μm atomic iodine amplifier systems by gas density changes, produced by gas-dynamic effects following nonuniform photolysis, has been analyzed for a wide range of line-broadening buffer gases. Mechanisms of photolysis gas heating and the relative merit of different buffer gases for minimizing density changes are discussed. An experimental study of the buffer gas dependence has shown good agreement with the gas-dynamic analysis. The model has been used to determine conditions for diffraction-limited amplification for varying pump times and buffer gas composition for typical large amplifier pumping profiles, showing the possibility for significantly increasing pump pulse duration.
Pulsed lasing from optically pumped rare gas metastable atoms (Ne, Ar, Kr, and Xe) has been demonstrated previously. The laser relies on a three-level scheme, which involves the (n+1)p[5/2](3) and (n+1)p[1/2](1) states from the np(5)(n+1)p electronic configuration and the metastable (n+1)s[3/2](2) level of the np(5)(n+1)s configuration (Racah notation). Population inversions were achieved using relaxation from ((n+1)p[5/2](3) to (n+1)p[1/2](1) induced by collisions with helium or argon at pressures near 1 atm. Pulsed lasing was easily achieved using the high instantaneous pump intensities provided by a pulsed optical parametric oscillator excitation laser. In the present study we examine the potential for the development of a continuous wave (CW) optically pumped Ar laser. We report lasing of the 4p[1/2](1)→4s[3/2](2) (912.547 nm) transition following CW diode laser excitation of the 4p[5/2](3)←4s[3/2](2) line (811.754 nm). A pulsed discharge was used to generate Ar 4s[3/2](2), and the time-resolved lasing kinetics provide insights concerning the radiative and collisional relaxation processes.
Our choice of rubidium atoms as the optically active species becomes now clear. The large elasticity of collisions between the electronically excited states of the rubidium D-lines and noble gas atoms has long been proven [25]. The quasi-molecular potential has a depth of the order of kBT, allowing for a significant part of the atoms’ kinetic energy to be extracted. Finally, the cooling transitions are in the near-infrared spectral regime, making them conveniently optically accessible to commercial laser systems. The cooling power Pcool from these experiments can be estimated in a similar way as for anti-Stokes cooling [26, 27] from the mean fluorescence frequency νfl, the incident power Popt, and the absorption probability of the gas mixture at incident frequency ν, α(ν).P Pcool opt fl= -a n n nn( ) (1.1) The typical energy that can be extracted from the sample per cooling cycle is of order of the thermal energy kBT. In the experiments performed so far, we observe cooling powers Pcool in the order of up to 100 mW [17, 23, 28, 29]. Note that it is important to choose a detuning with a high-enough absorption probability a(ν). We are aware that the collisional broadening plays a crucial role here. A measurement of typical fluorescence data recorded with the dense gas mixture will be shown in Section 1.4.1. Experimentally, it turns out to be crucial that the used buffer gas is of high purity to suppress the influence of concurring heating effects, of which quenching is the most known. In the case of quenching, the excitation energy is transferred to an atom of a third species polluting the dense gas mixture when colliding with the quasi-molecule. Relaxation can then occur via nonradiative decay, and so energy is kept in the system, heating the gas. To reduce possible quenching, an effect known to occur, for example, from an admixture with nitrogen, we use a buffer gas with high purity, namely argon gas preferably with a purity of 99.9999 (argon 6.0). Residual heating can also be caused by energy pooling. In this scenario, during the collision of two excited alkali atoms one of them gets further excited. Its relaxation to the ground state implicates the emission of a fluorescence photon with higher energy. For the case of rubidium atoms, the reaction equation of that process can be written as [30]: Rb Rb Rb Rb* * **+ + + Æ + + +mv mv mv mv12 22 12 22 2 2 2 2 . (1.2) The sum of the energies of the single excited rubidium atoms before the collision is higher than the energy of the twice excited atom. The surplus of the energy is thus kept in the system as kinetic energy of the collisional partners [31]. For rubidium atoms subject to a high-pressure noble gas, fluorescence originating from the 6p Æ 5s transition then occurs, and can be detected as blue light at a wavelength λ ≈ 420 nm in the experiments. At room temperature, the alkali atoms are in the solid phase. To produce a sufficient alkali vapor number density the system is heated to temperatures of up to 680 K, while the buffer gas pressure ranges between 50 bar and 230 bar. Under these conditions, we calculate that the dense gas ensemble consists of approximately 1017 cm-3 alkali atoms and 1021 cm-3 noble gas atoms. Then, we can estimate the mean free path length λfp for a gas of colliding atoms (cross section σ) and number density n l sfp = n (1.3) Let us consider the geometric cross section πd2, where d denotes the diameter of a rare gas atom, which is twice the van der Waals radius (d = 2rvdW = 3.76 × 10−10 m for argon [32]). Then, for argon pressure of 200 bar, the mean free path length is in the order of only a few nanometers, λfp ≈ 2.3 nm. Following the Boltzmann distribution, still considering a temperature T = 680 K, one can also estimate the collisional rate to 2.4 × 1011/s. Comparing this to the natural lifetime of the first excited alkali atom level (27 ns), we calculate that 104collisions occur within this period of time. 1.3 Experimental Setup and MethodsIn the following section, we present our experimental setup in detail, including the techniques used to measure atomic fluorescence and temperature changes inside the gas mixture. We offer insights into the high-pressure cells employed (Section 1.3.1), as well as into the spectroscopic techniques (Section 1.3.2) and the thermal deflection setup (Section 1.3.3).