The population and deactivation processes of the first excited 2S and 2D levels of alkali atoms have not been the focus of extensive attention to date, since these levels do not have dipole transitions to the 2S ground state, and the only radiative transitions to the 2p resonance level lie in the IR ( 1-3 fLm). We have demonstrated that in the case of potassium it is possible to populate the 32D and 52S levels in socalled optical collisions by laser excitation of the vapor and also by excitation energy transfer (EET) from the laser-excited Kidimers. 1 The lines of the corresponding 4D--->5P (1.529/1.475 f.lm and 6S ---> 5P (1.367/1.324 f.lm) transitions have also been identified in the laser-induced fluorescence of rubidium vapor,2 although the population mechanism of the levels is not examined in the present study. This st~dy will focus on determining the processes that result in the population of the Rb 42D and 62S levels and to determining the rate constants of these1'rocesses. The spectrum of the LGN-402 argon laser-induced fluorescence of Rb vapors was recorded in the IR by cooled Ge photo diodes selected for IR sensitivity and configured along the outlet slit of an MDR-3 monochromator. The synchronous detection method was employed. A FEU-79 photomultiplier was used in the visible range. Details on the experimental setup can be found in Ref. 3. The Rb vapor was excited by various argon laser lines (514.5-476.5 nm), although the 5l4.5-nm line was of primary interest. The C 1 ITu state of the RB2 dimer is not populated by this laser line; its predissociation efficiently populates the 52P resonance levels4 (see Fig. 1). Moreover the 5l4.5-nm laser line is closer in energy to the Rb 5S--->4D quadrupole transition (M= 75 cm-I). In the experiment at an atomic concentration [Rb] _1016 cm-3 the fluorescence lines from the 62S level (2.5 eV) were weaker by a factor of 50 compared to the lines from the 42D level (2.4 eV). A transition to the shorterwavelength laser excitation lines causes a certain increase in the population efficiency of the 4D level, although it also results in a more rapid growth of the population of the 5P resonance level due to the predissociation of Rb2 (C 1IT u ). I. In pure Rb vapors I) the line intensity of the 4D---> 5P transition has a linear relation to the laser excitation power ,(514.5 nm andp 5X 1016 cm-3 can be attributed to the onset of collisional quenching of the RB 4D state ('Trad =90 nsec). 5 So-called opticaJ.collisions lead to the population of the 4D level in these conditions (Aexc = 514.5 nm and no Na impurities)
The population and deactivation processes of the first excited 2S and 2D levels of alkali atoms have not been the focus of extensive attention to date, since these levels do not have dipole transitions to the 2S ground state, and the only radiative transitions to the 2p resonance level lie in the IR ( 1-3 fLm). We have demonstrated that in the case of potassium it is possible to populate the 32D and 52S levels in socalled optical collisions by laser excitation of the vapor and also by excitation energy transfer (EET) from the laser-excited Kidimers. 1 The lines of the corresponding 4D--->5P (1.529/1.475 f.lm and 6S ---> 5P (1.367/1.324 f.lm) transitions have also been identified in the laser-induced fluorescence of rubidium vapor,2 although the population mechanism of the levels is not examined in the present study. This st~dy will focus on determining the processes that result in the population of the Rb 42D and 62S levels and to determining the rate constants of these1'rocesses. The spectrum of the LGN-402 argon laser-induced fluorescence of Rb vapors was recorded in the IR by cooled Ge photo diodes selected for IR sensitivity and configured along the outlet slit of an MDR-3 monochromator. The synchronous detection method was employed. A FEU-79 photomultiplier was used in the visible range. Details on the experimental setup can be found in Ref. 3. The Rb vapor was excited by various argon laser lines (514.5-476.5 nm), although the 5l4.5-nm line was of primary interest. The C 1 ITu state of the RB2 dimer is not populated by this laser line; its predissociation efficiently populates the 52P resonance levels4 (see Fig. 1). Moreover the 5l4.5-nm laser line is closer in energy to the Rb 5S--->4D quadrupole transition (M= 75 cm-I). In the experiment at an atomic concentration [Rb] _1016 cm-3 the fluorescence lines from the 62S level (2.5 eV) were weaker by a factor of 50 compared to the lines from the 42D level (2.4 eV). A transition to the shorterwavelength laser excitation lines causes a certain increase in the population efficiency of the 4D level, although it also results in a more rapid growth of the population of the 5P resonance level due to the predissociation of Rb2 (C 1IT u ). I. In pure Rb vapors I) the line intensity of the 4D---> 5P transition has a linear relation to the laser excitation power ,(514.5 nm andp 5X 1016 cm-3 can be attributed to the onset of collisional quenching of the RB 4D state ('Trad =90 nsec). 5 So-called opticaJ.collisions lead to the population of the 4D level in these conditions (Aexc = 514.5 nm and no Na impurities)
Enhancement and reduction of the fluorescence intensity in Rb atomic vapor are observed. The reduction is due to a depopulation of the 5P - {3/2 } state upon illumination of a Rb vapor cell with light corresponding to the 5P - {3/2 }→5D - {5/2 } transition. Collisional transfer of the 5D - {5/2 } atoms to the 5D - {3/2 } state, followed by radiative decay to the 5P - {1/2 } state, creates an increase in the population of the 5P - {1/2 } state. The increase in the excited state population results in an increase in the fluorescence intensity of the 5P - {1/2 }→5S - {1/2 } transition. The laser-induced population changes are monitored by the decrease or increase of the corresponding fluorescence signals. Using the steady-state rate equations, the rate constants for the stimulated emission(i.e., 5D - {5/2 }→5P - {3/2 }) is 2.0×10 +8s + {-1 }. The cross section for the fine-structure changing collision(i.e., 5D - {5/2 }→5D - {3/2 }) is 6.4×10 + {-14 }cm +2.
Cross sections for $7^{2}D_{\frac{3}{2}}\ensuremath{\leftrightarrow}7^{2}D_{\frac{5}{2}}$ transfer in Rb, induced in collisions with He, Ne, Ar, and with ground-state Rb atoms, have been determined using methods of atomic fluorescence. Rb vapor, pure or mixed with a noble gas, was irradiated in a glass fluorescence cell with pulses of 660-nm radiation from a N2-laser-pumped dye laser, populating one of the 2D states by two-photon absorption. The resulting fluorescence included a direct component emitted in the decay of the optically excited state and a sensitized component arising from the collisionally populated state. Relative intensities of the components yielded the cross sections for 72D mixing: $Q(^{2}D_{\frac{3}{2}}\ensuremath{\rightarrow}^{2}D_{\frac{5}{2}})=8.8,6.5,10.4, \mathrm{and} 30$; $Q(^{2}D_{\frac{3}{2}}\ensuremath{\leftarrow}^{2}D_{\frac{5}{2}})=5.8,4.0,6.9, \mathrm{and} 18$, in units of ${10}^{\ensuremath{-}14}$ cm2 for He, Ne, Ar, and Rb, respectively. Cross sections for the effective quenching of the 2D states were also determined.
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).
We performed quasiresonant laser excitation of Li–Cd vapor mixtures at 460.3 nm (2p–4d) and 497.2 nm (2p–4s). The initial population of the Li(2p) level is caused by three processes: collisional dissociation or photodissociation of the Li 2 (B 1 Π u ) molecules or energy transfer in Li 2 (B 1 Π u ) and Li(2s) collisions. The Li(4d,4s) atoms radiatively decay towards Li(3p), from which collisional population transfer to Cd(5 3 P J ) occurs, followed by three-body reactive collisions which produce the LiCd(2 2 Π ) excimer states
Abstract. The cross sections for the Na.4D/! Na.4F / excitation energy transfer induced by collisions with He, Ar and Na ground-state atoms have been measured. The sodium atoms in the 4D state have been produced by two-photon excitation using cw laser radiation at 578.89 nm. Within a model which takes into account collisional population and depopulation of the 4F level as well as radiative relaxation (direct or cascade) of the 3D, 4P, 4D and 4F states ending in the 3P level, the rate for the Na.4D / ! Na.4F / collisional transfer turns out to be related to the ratio of the steady-state populations created in 3D and 4D states. They have been determined by measuring the fluorescence intensities of the 3D! 3P and 4D! 3P transitions. The measurements yield the following cross section values for those
A series of experiments is performed on cold 85Rb atoms confined in a magneto-optical trap. A variation of Stimulated Raman adiabatic passage (STIRAP) is used to transfer population from the 5S 1/2 ground state to the 5D5/2 state with an efficiency exceeding 80%. This process relies on moderately intense pulses arriving in the counterintuitive order, i.e., the upper transition (5P → 5D) driven before the lower transition (5S → 5P ). There is reasonable agreement with computer simulations, excepting a marked discrepancy between measured intensities and those used in the simulations. With a large ensemble of 5D atoms available, the 5D photoionization cross section at four different laser wavelengths (1064, 788, 647 and 532 nm) is determined via trap loss measurements. The theoretical calculations agree well with the experimental results (17.7, 10.9, 7.6 and 5.2 Mb, respectively). An experiment to measure the 5S-5D collisional rate constant sets an order-of-magnitude upper bound of 10−10 cm3/s, assuming all atoms are 5S-5D pairs. ^