Spin-orbit Relaxation Research Articles

Spin-orbit scattering in amorphous CuTi alloys

The authors present results for the temperature dependence of the electrical conductivity in various amorphous CuxTil-x alloys (x=0.35,0.425,0.50,0.65). It is shown that the data provide further evidence for the role of localisation and interaction effects in the description of electrical transport in disordered materials. A detailed comparison of theory and experiment provides estimates of the spin-orbit and inelastic relaxation times. A strong dependence of the spin-orbit relaxation time on Cu content is observed. The magnitudes of the spin-orbit relaxation times are consistent with simple calculations for pure Cu and pure Ti.

Mechanical torques generated by optically pumped atomic spin relaxation at surfaces

It is argued that a valuable method of observing certain types of surface-atom interactions may lie in mechanical torques generated through the spin-orbit relaxation of valence electronic spins of optically pumped atoms at surfaces. The unusual feature of this phenomenon is that the less probable spin-orbit relaxation becomes highly visible as compared with the much more rapid paramagnetic relaxation, because of an enhancement, typically by as much as a factor ${10}^{9}$, in the torques delivered to mechanical structures, by virtue of a very large effective moment arm. Spin-orbit relaxation operates through an exchange of translational momentum which, in turn, can be identified with the delivery of a gigantic angular momentum (in units of $\ensuremath{\hbar}$) relative to a distant axis about which mechanical motion is referred. The spin-orbit relaxation strongly depends upon the atomic number of the surface atoms and the strength of interaction with the optically pumped atoms. Being dominated by high-atomic-number surface atoms, spin-orbit-relaxation rates may not be too strongly influenced by minor surface contamination of lighter-weight optically active atoms.

Conduction electron paramagnetic resonance study of concentrated alkali metal-ammonia solutions

Conduction electron paramagnetic resonance was observed in concentrated alkali metal-ammonia solutions. The lineshape is Dysonian, and by analysis of the lineshape the diffusion time, relaxation time, and g shift were determined. In these solutions the relaxation time and g shift increase exponentially with the atomic number of the metal. The results of this study indicate that the spin-orbit relaxation mechanism is dominant. The relaxation time and g shift are discussed in detail and are compared to analogous studies on the alkali metals as well as other systems.

Curve-crossing collisions of excited lead atoms in flames

Lead atoms, present as a trace additive in a series of premixed H 2 –N 2 –O 2 flames, were excited to the 7 3 P o 1 state by 405.8 nm radiation from a nitrogen-pumped dye laser. Rate constants for spin-orbit relaxation to the 7 3 P o 0 state were obtained separately for collisions with atomic hydrogen and for collisions with the bulk flame gas, by measuring the relative intensities of fluorescence at 364.0 and 368.3 nm as a function of distance from the reaction zone in each flame. For hydrogen atoms the rate constant is typically 1 x 10 -9 cm 3 molecule -1 s -1 , decreasing with increasing temperature; for the bulk flame gas the rate constant is typically 1 x 10 -11 cm 3 molecule -1 s -1 , increasing with increasing temperature. Numerical calculations for model systems, with the use of Morse and Lennard-Jones potentials to describe the interaction of the colliding species, show that the negative temperature coefficient found for atomic hydrogen can be attributed to the crossing of attractive potential curves, corresponding to bound excited states of PbH.

Efficient multiphoton dissociation of CF3I+ in the metastable X̃ 2E1/2 excited state using cw infrared laser radiation

Evidence for the multiphoton dissociation of an electronic metastable state of CF3I+ with CO2 laser radiation is presented. Only those ions possessing sufficient internal excitation before irradiation can be dissociated. Photodissociation results primarily from resonant absorption by the ν1 vibrational mode of CF3I+ in the higher of the two, spin–orbit levels of the ground electronic state ? 2E1/2. Only the lowest energy decomposition channel is observed; this yields CF+3 and I. Spin–orbit and vibrational relaxation compete with photodissociation. At CF3I pressures below 2×10−7 Torr, the collisional relaxation rate is proportional to CF3I pressure kc = (3.1±0.4)×10−9 cm3 molecule−3 sec−1 with a zero pressure rate of 11.3±1.3 sec−1. The latter rate sets an upper limit on the spin–orbit relaxation rate. The photodissociation probability varies with laser frequency, exhibiting a broad, structureless peak near 960 cm−1. The initial photodissociation rate is proportional to laser irradiance kD = 0.80±0.08 sec−1/W cm−2. The photoproducts and laser frequency dependence are the same using either cw or pulsed irradiation. Ions were exposed to irradiances up to 120 W cm−2 cw and fluences up to 1.6 J cm−2 pulsed. Ion cyclotron resonance (ICR) techniques were used to store and detect the ions.

Laser induced reaction of I( 2p [formula omitted]) yielding I 2(B 3¶ ou+)

The photolysis of n-C 3F 7I in the uv yields I ( 2 P 1 2 ) with a quantum yield of nearly 1 (ref.1). The I( 2P 1 2 ) decay by spontaneous emission, in collisions with the n-C 3F 7I by spin-orbit relaxation, and by a three body reaction with the newly formed I( 2P 3 2 ), yielding I 2 (B 3¶ ou +). For the latter process the rate constant was determined to be k 3= (10±5) × 10 −33cm 6 molec. −2s −1 Further an upper limit of the rate constant for a possible bimolecular reaction of I( 2P 1 2 ) with n-C 3F 7I yielding I 2, (X 1σ og+) is given k 5= (3±1.5) × 10 −19cm 3 molec. −1s −1

Chemiluminescent iodine atom recombination

Photolysis of n-C 3F 7I at 266 nm produces metastable iodine atoms with a quantum yield of nearly 1. These particles have been observed to undergo spin-orbit relaxation in collisions with the parent molecules with a rate constant of (4.6 ± 0.3) × 10 −17 cm 3 molec −1 s −1. Excited and non-excited iodine atoms recombine in the presence of a third body to form I 2 (B 3π Ou +). The rate constant for this termolecular reaction is determined to be k 3 = (10 ± 5) × 10 −33 cm 6 molec −2 s −1 for the parent molecules as third bodies.

Mechanism of laser action due to 4d3D1,2–5p3P20transitions in a strontium atom

Investigations have been made of the energy and temporal characteristics of the 3066.2 and 3011.1 nm stimulated emission lines of a strontium vapor laser as a function of the discharge conditions. Experimental data are presented which confirm the hypothesis that the mechanism for depopulation of the lower active level of the lines under study is collisional spin-orbit relaxation in the 3Pj0 multiplet. It is concluded that the search for collisional laser action in atoms and ions exhibiting large multiplet splitting is promising.

Spin—orbit relaxation of Pb( 3P 01) by atomic hydrogen

Lead atoms in 7 3P 0 1 state were generated by 405.8 nm laser irradiation of metastable Pb( 3P 2) in a series of premixed H 2/N 2/O 2 flames. Fluorescence was observed from both the 3P 0 1 and 3P 0 0 states. The ratio of the intensities of fluorescence from the two states varied with flame parameters in a manner which indicated that spin—orbit relaxation of Pb( 3P 0 1) occurred rapidly in collisions with atomic hydrogen. The rate constant for this process is estimated as 3 × 10 −10 cm 3 molecule −1 s −1 at 1800 K.

Methyl isocyanide photodissociation: Chemical laser determination of energy partitioning into the cyanide radical photochemical product

Chemical laser techniques have been used to study the photodissociative excitation of cyanide radical by photolysis of methyl isocyanide: CH3NC→h/ωλ≳1550ÅCH3+CN* (A 2Πi) and to identify efficient spin-orbit relaxation of cyanide radical between its A 2Π1/2 and A 2Π3/2 manifolds. Many new chemical laser transitions have been obtained within the cyanide radical 0,0 red band [CN* (A 2Πi, v′=0) →CN(X 2Σ+, v″=0)] by selective cavity Q spoiling with a diffraction grating. In addition, measurements of the relative gain coefficients of grating-tuned laser transitions establish that photochemical reaction branching strongly favors CN* (A 2Πi, v=0) production; other possible product states [e.g., CN* (A 2Πi, v?1) and CN(X2Σ+, v=0)] are minor components (<10%) of the nascent cyanide radical concentration. The observed cyanide radical product state distribution is highly surprising (i.e., highly nonstatistical), signifying that strong dynamical effects govern the process of photodissociative excitation. All available spectroscopic, photochemical, and product energy partitioning data are used to formulate a state-to-state photochemical reaction mechanism for CN* (A 2Πi, v′) production by CH3NC photodissociation.

Vibrational excitation of N2 during optical pumping of Hg by 253.7 mμ resonance radiation

The observed decay time and the population density of the metastable 6 3P0 state of mercury in a N2:Hg gas mixture are markedly reduced with increased duration of an intense 253.7 mμ resonance radiation pulse. We show that this phenomenon is due to a vibrational excitation of the nitrogen in the spin–orbit relaxation decay of the 6 3P1 to the 3P0 state of Hg which is followed by the quenching of the 3P0 state. Such quenching takes place by diffusion and deactivation of 3P0 at the walls or by a three-body excimer formation in the gas. Observations are confirmed by a numerical solution of the rate equations for the different excited states in the system. The impact of these results on the performance of the optically pumped Hg laser is discussed.

Large isotope effect in the quenching of I(5 2P [formula omitted]) by benzene and benzene- d6

Spin—orbit relaxation of I(5 2P 1 2 )(Δ E = 0.94 eV) by benzene- d 6, has been studied at 297 K, using time-resolved atomic resonance fluorescence. A large isotope effect is observed, k C 6H 6 = (4.6 ± 0.7) × 10 −13 cm 3 molecule −1 s −1, and k C 6D 6 = (9.9 ± 1.0) × 10 −15 cm 3 molecule −1 s −1, despite evidence that formation of a bound collision complex may contribute to the quenching mechanism. The roles of resonant energy transfer channels, Franck—Condon factors and the density of final states, in the quenching process, are discussed.

Yields and quenching of I(5 2P [formula omitted]) following photodissociation of C 6H 5I and C 6F 5I

The yields of electronically excited iodine atoms I(5 2P 1 2 ) following the UV photodissociation of C 6H 5I and C 6F 5I have been studied using time-resolved atomic absorption spectrophotometry. The yields are shown to be low and spin—orbit relaxation by both molecules is rapid ( k(C 6H 5I) = (5.2 ± 0.4) × 10 −13, k(C 6F 5I) = (2.1 ± 0.2) × 10 −13 cm 3 molecule −1 s −1). The suitability of C 6H 5I and C 6F 5I as photochemical laser materials is discussed.

X-Ray Emission in Heavy Ion Collisions

In this paper a review will be given on three topics currently being studied through X-ray emission produced by atomic collisions with highly ionized projectiles. These are: 1) Mechanisms for nonstatistical substate populations deduced from measurement of the linear polarization of K X-rays. 2) Mechanisms for K vacancy production in atomic collisions deduced from K X-ray–scattered particle coincidence measurements. 3) Multiple K vacancy production and sharing, and decay as deduced from X-ray-X–ray coincidence experiments. K X-ray polarization data will be presented to support relatively simple models of preferential substate populations predicted for ionization, excitation, and electron capture processes. The importance of spin-orbit relaxation and the effect of cascades on the observed X-ray polarization has been experimentally investigated and will be reviewed. Data showing the impact parameter dependence of inner shell vacancy production by direct ionization, 2pσ-2pπ rotational coupling, and electron capture will be presented. Disagreements with current theoretical models have been documented and will be emphasized. The large probability for inner shell vacancy production in heavy ion collisions makes the study of multiple vacancy decay by photon-photon coincidence techniques particularly attractive. Use of both real and random coincidence events provide Direct experimental measurement of multiple vacancy production, hypersatellite spectroscopy, vacancy sharing between separating atoms, and branching ratios between competing decay channels.

Conduction electron spin resonance in gold

Conduction electron spin resonance has been observed in transmission through pure gold slabs of 100, 260, and 490 µm thickness covered on each side by a thin ferromagnetic film. This resonance corresponds tog=2.11±0.01 and broadens rapidly above 10 K, in agreement with spin orbit relaxation by phonons.

Temperature dependence of the spin–orbit relaxation of lead, 6p2(3P2) and (3P1)

Rate constants for the collisional deactivation of Pb 6p2(3P2) and (3P1) metastable states are reported over the temperature range 300 to 600 °K. Quenching data over this temperature range were obtained for relaxation by Ar, Xe, H2, D2, N2 and by Hg at 373 °K. These results were obtained in a temperature controlled, flash photolysis apparatus using tetramethyl lead as a precursor and time resolved atomic resonance absorption techniques for monitoring the 3P2 and 3P1 metastable states of interest.

Isotope effects in the quenching of electronically excited atoms. Part 4.—Quenching of I(52P½) by hydrogen and deuterium halides, H2O and D2O

Spin-orbit relaxation of I(52P½) by the molecules H2O, D2O, HCI, DCl, HBr, DBr, HI and DI has been studied using time resolved atomic absorption spectrophotometry (λ= 206 nm; T= 293 K). It is concluded that quenching by the two most efficient molecules, H2O and HBr, proceeds via resonant energy transfer channels (electronic → vibration + rotation) probably involving quadrupole–dipole coupling. However, the small isotope effects and relatively large cross-sections observed for the other molecules suggest that an efficient curve crossing mechanism dominates for HCI, DCI, DBr, HI and DI. This is consistent with other recent work involving Br(42P½) and the hydrogen halides.

Phase-shift studies of Hg(3P0) reactions. Part 5.—Kinetics of Hg2excimer reactions in the presence of nitrogen

A modulated source of 253.7 nm mercury resonance radiation has been used to excite Hg2 excimer bands in mixtures of mercury vapour (at a fixed pressure of 0.27 Torr) and nitrogen at pressures between 1 and 200 Torr. Phase shifts of the emission bands at 335 and 485 nm, relative to 253.7 nm atomic fluorescence, have been measured as functions of nitrogen pressure at 373 K. The results are consistent with a mechanism in which Hg(3P0), produced by spin-orbit relaxation of Hg(3P1) in collision with nitrogen, reacts in a termolecular reaction with a ground-state (1S0) mercury atom and a nitrogen molecule to form Hg2(31u). The Hg2(31u) then either radiates the 335 nm band or is converted to Hg2(30–u) by collision with nitrogen (k7= 4.8 ± 0.5 × 10–15 cm3 molecule–1 s–1). The 485 nm emission arises, under the present experimental conditions, exclusively from a collision-induced process involving Hg2(30–u) and a nitrogen molecule (k10= 5.7 ± 0.3 × 10–15 cm3 molecule–1 s–1). The effects of wall processes are of minor importance except at the lowest pressures used.

Laser induced photodissociation of iodine molecules. Spin-orbit relaxation of I( 2P [formula omitted

A flash lamp pumped dye laser is used to dissociate I 2 molecules in the wave-length region of the B state continuum. Excited I atoms are produced in the 2P 1 2 state. The quenching of these excited atoms in the presence of I 2 molecules was measured and the rate constant was found to be (1.79 ± 0.07) × 10 10 ??? mol −1 s −1.

Vibrational excitation of N2 by the spin-orbit relaxation of Hg(63 P 1? 63 P 0)

The lifetime of Hg(63P0) in a gas mixture of Ar + N2 at room temperature was measured by a phase-shift method. A mixture containing a trace of Hg vapour was illuminated by 253.7 nm radiation, whose intensity was modulated at a frequency in the range of 120–1000 Hz, and the resulting a.c. component of Hg(63P0) concentration was monitored by a lock-in system. The lifetime is inversely proportional to the square root of the 253.7 nm radiation intensity. This is attributable to accumulation of N2 in the v= 1 level through the spin-orbit relaxation Hg(63P1)+ N2(v= 0)→ Hg(63P0)+ N2(v= 1), and the reverse of this reaction results in shortening of the lifetime. The accumulation of N2 in the v= 1 level is due to the fact that N2(v= 1) is deactivated by a factor of about 102 less efficiently than Hg(63P0). The observed dependence of the lifetime on the 253.7 nm intensity made it possible to deduce the rate constant of the relaxation. The result agreed with the value determined from the lifetime measurement of Hg(63P1) in N2. Thus, it is concluded that the spin-orbit relaxation of Hg(63P1) in N2 consists of the electronic-to-vibrational energy transfer. The reciprocal lifetime of N2(v= 1) was determined to be 1.2 ± 0.3 s–1 in the mixture of 1 Torr N2+ 9 Torr Ar. This leads to the estimate that the collision probability for deactivation of N2(v= 1) on the wall of a quartz cell is 1 × 10–4.