Multistep Excitation Research Articles

Role of metastable states in circumventing laser pulse jitters in multi-step laser excitation experiments

The role of metastable states in circumventing laser pulse jitters in multi-step laser excitation of high-lying Rydberg and autoionizing states is discussed. A case study is presented where step-wise excitation of 3d4d 3G 3,4,5 states of Ca is achieved despite a large jitter between the laser pulses.

Pressure dependence of photoluminescence in amorphous silicon nanopowder produced by plasma enhanced chemical vapour deposition

The photoluminescence emission in silicon powder grown by plasma enhanced chemical vapour deposition has a very unusual dependence on pressure. Its intensity diminishes exponentially, I PL I 0 exp(- P/P 0 ), where P 0 has a value of several pascals. Consequently it is detectable only under vacuum. This dependence is analysed within the framework ofa multistep multiphoton excitation process. It is shown that an exponential dependence on pressure of the dynamic constants (lifetimes and optical cross-sections involved in the model) can account qualitatively for all the experimental results. MST/3288

Constant-electric-field ionization mass spectroscopy in laser-excited Yb^+ Rydberg states

Using a new method of constant-electric-field ionization of Rydberg states of ions, we have eliminated the influence of the direct multiphoton ionization background in multistep excitation of Yb+ Rydberg states and obtained the pure spectrum of np(n = 17−86) Rydberg levels of Yb+.The precision of the value of the Yb ii ionization potential has been greatly improved to 98 231.75(20) cm−1.

Instrumentation for multistep excitation of lithium atoms to Rydberg states

We have developed a diode laser apparatus to excite Li from its ground 2S state, through 2P and 3S, to its Rydberg states with three cw diode lasers operating at λ = 671 nm, 813 nm, and 630-635 nm. A He-Ne laser at λ = 633 is sometimes used in place of the 635-nm diode laser for the last step. The output power of each of these lasers was ~1 mW. We describe our technique of locking the first two lasers on Li resonance lines by obtaining a fluorescent signal from the second decay (3S ? 2P) that is normally overpowered by a strong background of fluorescent light from the first decay (2P ? 2S). We used two balanced photodiodes to reject the strong fluorescent light without loss of collection efficiency. A rejection ratio as high as 100 has been obtained.

Population kinetics of lithiumlike ions in plasma.

For lithiumlike ions (oxygen and aluminum), characteristics of the population distribution over the excited levels and the population kinetics are examined in detail. In the high-density region, a simple approximate distribution function is derived for both the ionizing and recombining plasma components on the basis of the multistep ladderlike excitation or deexcitation mechanism, combined with the thermodynamic equilibrium distribution. As examples of applications, we present intensity ratios of emission lines from impurity ions in hydrogen plasma, and the population inversion and gain for the x-ray recombining plasma laser.

Optical measurements of core density fluctuations in TEXT-U using a helium metastable beam

A compact, portable, eight-channel, filter spectrometer has been designed for beam emission spectroscopy measurements of long-wavelength density fluctuations on TEXT-U and Phaedrus-T. The system uses radially elongated image volumes which maximize the étendue of the system while still resolving the dominant fluctuations (kr<2 cm−1, kθ<6 cm−1). Because of the vertical injection geometry of the TEXT-U diagnostic neutral beam, there is little Doppler shift of the beam emissions and thus a nonhydrogenic species must be used to distinguish between the beam and edge emissions. A He0 metastable beam has been found to provide good beam penetration with little contamination of the detected beam fluorescence by edge emission. A multistep excitation model is used to quantitatively interpret the fluctuations in the intensity of the He0 triplet (3P0–3D, 587.6 nm) as plasma density fluctuations.

Hematoporphyrin-sensitized degradation of deoxyribose and DNA in high intensity near-UV picosecond pulsed laser photolysis

The photosensitized degradation of deoxyribose and DNA, using hematoporphyrin (HP) and picosecond laser pulses at high intensities (pulse duration 30 ps, λ exc = 355 nm, light intensity range 10 8–10 10W/cm 2) was studied. Aldehyde formation from 2-deoxy- d-ribose and long-chain double-stranded DNA, when analyzed as a function of light intensity, followed a non-linear dependence, suggesting the involvement of multiphoton light absorption by HP. The degradation mechanism was studied by analysis of the yield dependence on excitation intensity and the effect of added radical scavengers. The participation of OH radicals in the degradation process was confirmed by spin trapping techniques. At low light intensities added N 2O largely increased product formation, suggesting that HP photoionization predominates under these conditions. At higher intensities (I ≥ 3 GW/cm 2) the product yield was not affected by N 2O which, combined with spin trapping data, suggested that OH radical formation occurred, but that neither HP photoionization nor peroxy radical formation was involved. Single and double strand breaks in supercoiled plasmid DNA (pBR 322) confirmed the generation of OH or OH-like radicals during high-intensity excitation of HP. A mechanism involving a multistep excitation of HP, followed by resonance energy transfer to H 2O resulting in dissociation to yield OH and H atoms, is proposed.

SUBTHRESHOLD ANTIPROTON PRODUCTION IN HEAVY-ION COLLISIONS

We present a RQMD calculation of antiproton yields and their momentum distribution in Ne+NaF collisions at 2 GeV /u. The antiprotons can be produced below threshold due to multi-step excitations for which meson-baryon interactions play a considerable role. In this system the annihilation probability for an initially produced antiproton is predicted to be about 65%.

Multipole structure in asymmetrical double Rydberg states.

We have investigated Ba high-l 8snl double Rydberg states produced by multistep laser excitation in the presence of a temporarily applied electric field to probe the Coulomb interaction between the two excited electrons. The spectra for the 6s\ensuremath{\rightarrow}8s excitation with n=13 (l=12 to 6) show a strong dependence on the orbital momentum of the external electron, due to multipole effects. To explain the 5fn'l' resonances appearing in the n=13 and 14 l=6 spectra, terms higher than the dipole interaction have to be considered in the multipole expansion.

An experimental investigation of the 6 sns and 6 snd doubly excited autoionizing states of strontium

Eight doubly excited 6 sns and 6 snd autoionizing levels of strontium were detected by using the laser multistep excitation technique. The results agree with the isolated core excitation model.

Investigation of lutetium rydberg states by laser multistep resonance ionization spectroscopy

Laser multistep excitation and electric-field ionization spectroscopy have been used to investigate experimentally highly excited states (HES) of lutetium in the vicinity of the first ionization limit. The investigation includes the measurement of energy levels (ionic signal vs last transition frequency) and radiation lifetime (ionic signal vs ionizing electric-field pulse delay) of the states investigated. Even Rydberg states of 4f14 6s2nd have been observed with two-step laser excitation. The maximum experimental error is 0.3 cm−1 for the energy and 20% for the radiation lifetime measurements. This is the very first time that results for the lifetimes as well as for a large part of the energy values have been obtained. Our present experimental results compare well with previously calculated values obtained by relativistic perturbation theory using the zero-order model approximation, and with the available experimental values.

Rydberg states of the strontium ion

We report on the spectroscopy of highly excited strontium ions after resonant multistep laser excitation. The states observed belonged to then s (26≦n≦79),n d (25≦n≦83),n f (28≦n≦86) andn g (30≦n≦81) series of Sr+. A least squares fit has been applied to determine the quantum defects and their linear energy dependence in each series, together with the ionisation limit of Sr+. The analysis of our data lead to a revised value for this ionisation limit of 88 965.18±0.02 cm−1, with respect to the ground state of the Sr+ ion. Analysis of the non core-penetratingn g states yielded experimental values for the dipole and quadrupole polarizability of Sr++, which are in satisfactory agreement with previous calculations.

High angular momentum 6pnl and 6dnl doubly excited Rydberg states of barium

6p32/nl (n=11 to 15, l=5 to n-1) and 6d52/nl (n=11 to 14, l=6 to n-1) doubly excited Rydberg states of barium have been studied using a multistep laser excitation technique. The excitation is via the bound 6s12/nl states which are populated using an electric-field switching method which allows the selective population of high-l angular momentum states. The resulting spectra clearly demonstrate the presence of dielectronic correlation. The authors show that this correlation is well described in terms of the (j-l)K coupling scheme, taking into account in a perturbative way the dipole and quadrupole interaction terms.

Double circular states of barium.

We have populated the double circular Ba 4f5g levels using multistep laser excitation from the ground state. The observed autoionization rates of these levels are quite low, which may indicate strong electron correlations. We present a semiclassical picture that explains the long lifetimes of these states without quantum-mechanical correlation effects. In addition to the 4f5g states, we have also observed higher-lying 4fng states. In particular, we have observed the mixing of the bound 4f6g and 4f7g levels with the 6d(j)nf and 6d(j)nh series indicating direct orbital angular momentum exchange between the two valence electrons. We observe orbital angular momentum exchange between bound electrons in which the initial and final states of both electrons can be identified.

ANALYZING TRACE ELEMENTS BY RIS-TOF MASS SPECTROMETRY

A new method of laser resonance ionization spectroscopy combined with time-of-flight mass spectrometry (RIS-TOF) has been investigated to detect trace amounts of elements in a substance. It is based on thermal atomization of a substance in vacuum and detection of the trace atoms by the laser resonant ionization technique. A rather effective way of detecting atoms is by multistep excitation to autoionization states with laser beams. The measurement results of Au and Ru in solution samples and in geological samples are described. The direct detection limit of Au and Ru is 0.1 ppb in a 10 μl solution sample.

Laser resonant photoionization spectroscopy of highly excited and autoionization states of terbium atoms

The energies of unknown highly excited states of terbium atom have been measured by multistep laser excitation followed by photoionization: 19 odd and 25 even states in the 32300-35400 cm-1 energy region with an accuracy of +or-0.3 cm-1 and 135 odd and 85 even autoionization states in the 47800-52700 cm-1 energy region with an accuracy of 3 cm-1 were investigated. The optimal three-step schemes of Tb atom photoionization through autoionization states in the yellow-green region of the spectrum have been defined.

The (5d3/2,5/2; np and nf) J=1 series in atomic barium

The authors report results, obtained by a multi-step laser excitation technique, on the (5d3/2,5/2; np and nf) J=1 autoionizing states in barium. The observed series exhibit noticeable electronic channel interactions. The agreement between these results and a theoretical approach that combines eigenchannel R-matrix and multichannel quantum defect methods gives further support to this approach.

Spectroscopy of autoionizing states of the strontium atom in photoprocesses

The characteristics of the lowest 1P autoionizing states of the Sr atom, which are excited by a weak electromagnetic field, and also the parameters, which determine the electronic and radiative decay (harmonic generation) of these states under multistep laser excitation through the intermediate discrete states 5s5d 1D and 5p2 1D, are determined. The line profiles of the resonances in the ejected photon and electron spectra are obtained. The calculation is carried out in the diagonalization approximation, the state classification is based on the LS-coupling scheme and the coupling of photon and electron decay channels is allowed for. The theoretical results agree with the experimental data on the excitation of the 1P autoionizing states obtained using linear and non-linear optics.

6pnf (J=2, 3, 4) and 6pnh (J=4, 5, 6) autoionising series of Ba

The autoionising series 6pnf (J=2, 3, 4) and 6pnh (J=4, 5, 6) of barium have been studied in an atomic beam using multistep laser excitations from the 6s5d and 5d2 1G4 metastable states. Level assignments were made assuming jK coupling and using calculations of electric quadrupole splittings resulting from the long-range electrostatic interaction as well as calculated oscillator strengths. Effects of the interactions between the 6p1/2nf, h and 6p3/2 nf,h series as well as between the 6pnf and 6pnp configurations could be observed.

Ba 6pjnfj' autoionizing series.

Members of the 6${p}_{j}$${\mathrm{nf}}_{j\mathcal{'}}$ autoionizing series in barium have been observed experimentally using multistep laser excitation. The recorded spectra exhibit extremely broad profiles for n values ranging from 7 to 30. We have fitted the spectra using an energy-independent multichannel quantum-defect theory model that reproduces the data reasonably well in spite of the fact that the full widths at half maximum of the excitation profiles are comparable in magnitude to the energy spacings between adjacent states. Experimental quantum defects and autoionization rates are presented along with the parameters determined from the theoretical fit.