Isotope-selective Excitation Research Articles

Possibility of resonant three-photon isotope-selective excitation of vibrational states with υ = 3 of the UF6 molecule mode ν 3 by radiation from two pulsed IR lasers in a gas-dynamically cooled molecular flow

We consider the possibility of resonant isotope-selective population of states with υ = 3 of the vibrational mode ν 3 of UF6 molecules as a result of three-photon bichromatic excitation of the molecules by IR radiation from two pulsed CF4 lasers and two pulsed para-H2 lasers. We propose specific schemes and analyse the possibility of exciting the 238UF6 and 235UF6 molecules into states with υ = 3 of the vibrational mode ν 3 using two CF4 lasers (or para-H2 lasers) generating in the region of 16 μm at frequencies that are significantly (by ∼3.5 – 17 cm−1) detuned from the linear absorption bands at the frequencies of ν 3 vibration of these molecules in a gas-dynamically cooled molecular flow. However, the sum of the doubled lasing frequency 2ν L1 of the first laser and the frequency ν L2 of the second laser is equal to a frequency of the transition υ = 0 → υ = 3 of the vibrational mode ν 3 of the UF6 molecules. When both laser pulses coincide in time, the possibility of selective excitation of UF6 molecules from the ground vibrational state with υ = 0 to the state with υ = 3 of the vibrational mode ν 3 is realised. The proposed schemes for the excitation of UF6 molecules to states with υ = 3 are compared with the previously implemented schemes for the effective excitation of states with υ = 3 of the vibrational mode ν 3 (F 1) and states with υ = 2 of the vibrational mode ν 3 (A 1) of SF6 molecules by three- and two-frequency radiation of pulsed CO2 lasers, respectively.

Selective photoionization of palladium isotopes using a two-step excitation scheme

We present a novel two-step even–odd mass isotope selective excitation and ionization scheme, potentially applicable in resource recycling and management of palladium occurring in high-level nuclear waste. In contrast to the conventional three-step selective ionization process, the two-step scheme utilizes transition selection rules to an autoionizing Rydberg state, rather than to an intermediate state, resulting in an increase in efficiency of over an order of magnitude while retaining excellent selectivity of >99.7%. The reduction in the number of excitation lasers required allows several technical simplifications and reduces costs should the process be developed for large-scale resource recycling operations.

Quantum optimal control of the isotope-selective rovibrational excitation of diatomic molecules

We carry out optimal control theory calculations for isotope-selective pure rotational and vibrational-rotational excitations of diatomic molecules. The fifty-fifty mixture of diatomic isotopologues, 7Li37Cl and 7Li35Cl, is considered and the molecules are irradiated with a control pulse. In the wave packet propagation we employ the method quantum mechanically rigorous for the two-dimensional system including both the radial and angular motions. We investigate quantum controls of the isotope-selective pure rotational excitation for two total times 1280000 and 2560000a.u. (31.0 and 61.9ps) and the vibrational-rotational excitation for three total times, 640000, 1280000, and 2560000a.u. (15.5, 31.0, and 61.9ps) The initial state is set to the situation that both the isotopologues are in the ground vibrational and rotational levels, v=0 and J = 0. The target state for pure rotational excitation is set to 7Li37Cl (v=0, J = 1) and 7Li35Cl (v=0, J = 0); that for vibrational-rotational excitation is set to 7Li37Cl (v=1, J = 1) and 7Li35Cl (v=0, J = 0). The obtained final yields are quite high and those for the longest total time are calculated to be nearly 1.0. When total time is 1280000a.u., the final yields for the pure rotational excitation are slightly smaller than those for the vibrational-rotational excitation. This is because the isotope shift (difference in transition energy between the two isotopologues) for the pure rotational transition between low-lying levels is much smaller than that for the vibrational-rotational transition. We thus theoretically succeed in controlling the isotope-selective excitations of diatomic molecules using the method including both radial and angular motions quantum mechanically.

Studies on the Optogalvanic Effect and Isotope-Selective Excitation of Ytterbium in a Hollow Cathode Discharge Lamp Using a Pulsed Dye Laser

This paper presents studies on the pulsed optogalvanic effect and isotope-selective excitation of Yb 555.648 nm (0 cm(-1) → 17 992.007 cm(-1)) and 581.067 nm (17 992.007 cm(-1) → 35 196.98 cm(-1)) transitions, in a Yb/Ne hollow cathode lamp. The Yb atoms were excited by narrow linewidth (500-1000 MHz) Rh110 and Rh6G dye based pulsed lasers. Optogalvanic signal inversion for ground state transition at 555.648 nm was observed beyond a hollow cathode discharge current of 8.5 mA, in contrast to normal optogalvanic signal at 581.067 nm up to maximum current of 14 mA. The isotope-selective excitation studies of Yb were carried out by recording Doppler limited optogalvanic signals as a function of dye laser wavelength. For the 581.067 nm transition, three even isotopes, (172)Yb, (174)Yb, and (176)Yb, and one odd isotope, (171)Yb, were clearly resolved. These data were compared with selective isotope excitation by 10 MHz linewidth continuous-wave dye laser. For 555.648 nm transition, isotopes were not clearly resolved, although isotope peaks of low modulation were observed.

Isotope-selective laser excitation and field ionization of the nF5/2 Rydberg states in a thallium beam

The effective isotope-selective excitation and field ionization of the 16F5/2 thallium Rydberg states have been studied experimentally. The 16F5/2 states were excited by the two-step scheme 6P1/2→6D3/2→nF5/2 in a dense collimated atomic beam by means of monochromatic radiation from two frequency tunable lasers (λ1 = 276.9 nm, λ2 = 773–808 nm) with a mean power density of more than 1 W. The largest selectivity of excitation in this scheme takes place in the first step 6P1/2→6D3/2. The experiments showed that the main reason for the limiting selectivity excitation of the Rydberg states is the field broadening of the absorption resonances at the first and second steps of the excitation scheme. The excitation efficiency of this scheme is considerably governed by the parameters of the laser radiation at the second step and an electric field pulse.

Calculation of absorption contours and excitation selectivity of isotopic molecules 12CO and 13CO for implementation of photocatalytic reactions

The possibilities of the isotope-selective excitation of molecules by CO laser radiation. More specifically, the problem of determining the conditions of the preferable excitation of heavy isotope 3C18O compared to the light isotope 12C18O is set. A carbon monoxide molecule is capable of absorbing at hundreds of vibrational-rotational lines within the principal absorption band of the CO molecule over the 1200–2600 cm−1 range. Since radiation absorption is negligibly low in the frequency range below 1900 cm-1 and there are no appreciably efficient radiation sources over the 2200–2600 cm−1 range in which absorption can be observed, the 1900–2200 cm−1 frequency range is considered in this work. The presented calculations may refer to the selective excitation of the gas phase in bulk and in gas absorbed on the surface.

Quantum control study of multilevel effect on ultrafast isotope-selective vibrational excitations

Quantum optimal control calculations have been carried out for isotope-selective vibrational excitations of the cesium iodide (CsI) molecule on the ground-state potential energy curve. Considering a gaseous isotopic mixture of (133)CsI and (135)CsI, the initial state is set to the condition that both (133)CsI and (135)CsI are in the vibrational ground level (v=0) and the target state is that (133)CsI is in the v=0 level while (135)CsI in the first-excited level (v=1). We find that, using the density-matrix formalism, perfect isotope-selective excitations for multilevel systems including more than ten lowest vibrational states can be completed in much shorter time scales than those for two-level systems. It is likely that this multilevel effect comes from the large isotope shifts in the vibrational levels of v>1. To check the reliability of the calculation we also carry out optimal control calculations based on the conventional wave-packet formalism, where the wave-function amplitude is temporally propagated on the grid points in real space, and obtain almost the same results as those with the density-matrix formalism.

Multilevel effect on ultrafast isotope-selective vibrational excitations: Quantum optimal control study

We demonstrate, using optimal control theory, that perfect isotope-selective molecular vibrational excitations on the ground-state potential curve in multilevel systems can be completed in much shorter time scales than those in two-level systems. We consider a gaseous isotopic mixture of cesium iodide ( 133CsI and 135CsI) and, using two-level and multilevel systems, try to obtain electric fields that drive different isotopes into different vibrational levels. As a result, we find that in multilevel systems isotope-selective excitation processes can be controlled much faster, which we call multilevel effect. It is likely that this effect makes use of the large isotope shifts of higher vibrational levels than the lowest two.

Isotope selective near-resonant two-photon ionization of ^41Ca isotope via the 4s11s ^1S_0 intermediate state

Isotope selective excitation of a 41Ca isotope using a near-resonant two-photon ionization scheme 4s2 1S0-->422.7924 nm,413.3685 nm4s11s 1S0-->514.5 nmCa+ has been proposed for using the 41Ca isotope in applications as a tracer in biomedical studies. The ionization efficiency and optical selectivity have been calculated for various powers of the excitation and ionization laser. Under the optimized excitation and ionization laser powers the ionization efficiency for the studied scheme is found to be 1.7x10(-4). The optical selectivity value is approximately 1.0x10(5) and both of these values are either comparable or slightly better than the earlier published work by our group. The overall ionization efficiency for the two-photon ionization scheme considering the throughput factor is 5x10(-3), which is 2 orders of magnitude higher than the stepwise excitation process. Therefore, the higher ionization efficiency of the process enables monitoring of the tracer isotope for longer durations. In combination with a mass spectrometer, an abundance sensitivity of approximately 10(10) can be obtained, which is adequate for biomedical applications.

Isotope selective near-resonant two-photon ionization of calcium isotopes

A near resonant two-photonionization scheme , has been investigated for the isotope selective excitation of 41Ca isotope for its applications as a tracer in bio-medical studies. The ionization efficiency and optical selectivity have been calculated for various powers of the excitation and ionization laser. Under the optimized excitation/ionization laser powers the ionization efficiency and the optical selectivity for the studied scheme are found to be 1.3 × 10−4 and ∼1.0 × 104, respectively. The ionization efficiency is two orders of magnitude higher than the step-wise excitation process. In combination with a mass spectrometer, an overall selectivity of ∼1010 can be obtained which is adequate for bio-medical applications.

Isotope-selective excitation of ^41Ca isotope in Doppler-free two-photon continuous-wave excitation: a case study

Seven schemes are studied theoretically for Doppler-free two-photon excitation of rare (41)Ca isotope using single-mode continuous-wave lasers. The ionization efficiencies and optical selectivities for all the schemes are calculated for various powers of the excitation and ionization lasers and for various focusing conditions of the two lasers. To maximize the ionization efficiencies and the optical selectivities, wavelength-dependent Stark compensation is used. Certain laser wavelengths of the ionization step termed as magic wavelengths are identified for compensating the Stark shift induced by the excitation laser. The effects of the Stark-shift-induced asymmetry and its reversal by selecting the appropriate magic wavelength for the ionization step for various excitation and ionization laser intensities are investigated. The ionization efficiency and optical selectivity for the best scheme after Stark compensation are found to be 8.4 x 10(-4) and approximately 9 x 10(3), respectively.

Effect of 87Sr hyperfine structure on the isotopeselective excitation of 89Sr and 90Sr isotopes incollinear resonance ionization spectroscopy

The 87Sr isotope has an extremely complex hyperfine structure(HFS) as a consequence of simultaneous mixing of the 3F2, 3F3, 3F4 and 1F3 fine structurecomponents in the 5s32f state. The resultant hyperfine spectrumof the 5s4d 3D3-5s32f 3F4 (364 nm)transition consists of 74 hyperfine components. HFSconstants A, B for both upper and lower levels havebeen derived from the single-electron contribution factors a5s, a4d3/2, a4d5/2, b4d3/2 and b4d5/2 using Breit-Wills formalism.Isotope ratio enhancement factors have been computed for the5s5p 3P2-5s21s 3S1 (325 nm),5s4d 3D3-5s32f 3F4 (364 nm) and the5s2 1S0-5s5p 3P1 (689.7 nm) transitions forisotope selective excitation/ionization of 89Sr and90Sr isotopes in collinear fashion. The results obtainedare comparable to that of the experimental results reported earlier. Simulations have been carried out for various laserlinewidths and acceleration potentials considering appropriateabundances, HFS of the odd isotope and massshifts of the constituent isotopes. This method can be used foroptimization of experimental configurations with respect toacceleration potentials and appropriate laser linewidths in orderto obtain better selectivities for the 89Sr and 90Srisotopes which allows the long term monitoring of theseisotopes in the environment. The present study shows that theHFS of the 87Sr isotope has a significanteffect on the isotope selective excitation of 89Sr and90Sr isotopes.

Abundance ratios of mercury isotopes by 6 s 6 p 3 P 1 -intermediate state excitation with resonance ionization mass spectrometry

The isotope-selective excitation of mercury via the 6s6p3P1 intermediate state excitation is studied with two-color resonance ionization mass spectrometry to determine abundance ratios of stable mercury isotopes. Lifetime, isotopic shifts, and hyperfine structure (HFS) splittings are measured. Atomic hyperfine interaction constants are determined. Line strengths of the ten components of the 6s21S0→ 6s6p3P1 transition are used for isotope ratio determinations. Ion signal intensities of even-mass components directly give the isotopic abundance, in contrast to the odd-mass components, for which the sum of the HFS components results in an anomalously high response for the isotopic abundance. The laser bandwidth dependence of the excitation and population probabilities of magnetic HFS sublevels due to linear polarization of the radiation are discussed. These considerations yield reasonable values for the apparent odd-mass isotope abundance.

Isotope selective excitation of 155Gd and 157Gd isotopes from 9D° 2–6 states using broadband lasers

The feasibility of isotope enrichment of 155Gd and 157Gd isotopes using relatively broadband lasers within the tuning range of rhodamine-6G dye has been studied. A new set of photoionization schemes have been proposed for using in the industrial scale enrichment of these isotopes. These photoionization schemes result in better photon economy compared to the photoionization schemes reported earlier. The enriched isotopic mixture results in a 50% increase in the neutron absorption cross-section and a 70% reduction in the residual neutron absorption. The degree of enrichment of the odd isotopes is adequate for using the enriched mixture as an efficient burnable poison. The effect of linewidth of the excitation laser and Doppler width of the atom source on the degree of enrichment have been theoretically evaluated and experimentally verified. The computed selectivities were in good agreement with the experimental results.

Determination of 6Li/ 7Li isotope ratio using two photon resonance three photon resonance ionisation mass spectrometry

Isotope selective excitations limit the measurements of isotope ratios using resonance ionisation mass spectrometry. We have used spectral simulation method for estimation of isotope selectivities and isotope ratio enhancement factors by taking into account the isotope shifts, bandwidth of the excitation laser and abundance of the constituent isotopes. Isotope selectivity calculations have been performed for natural abundance lithium for various laser bandwidths up to 5000 MHz and residual Doppler widths for the 2s 2S 1 2 -4s 2S 1 2 (571.0833 nm) two photon transition, the 2s 2S 1 2 -4s 2p 2p° 3 2 (670.791 nm) and the 2s 2S 1 2 -4s 2p 2p° 3 2 (670.776 nm) single photon transitions. The results obtained for the 2s 2S 1 2 -4s 2p 2p° 1 2 (571.0733 nm) transition were found to be in good agreement with the theoretical results reported earlier. The computed isotope ratio enhancement factor along with the experimentally determined isotopic selectivity were used for determination of isotope ratio and abundance of the constituent isotopes. Abundance of the constituent isotopes and the isotope ratios measured with RIMS and ICP-MS agree quite well. The method demonstrates the utilisation of resonance ionisation mass spectrometry for isotope ratio measurements despite differences in ionisation of various isotopes due to laser wavelength, linewidth and Doppler widths.

Studies on the isotope selective photoionization of the low-abundant isotope

The efficiency of the two photoionization schemes and has been studied by a spectral simulation approach for the isotope-selective excitation of the isotope. Isotopic selectivities have been computed for (555.648 nm), (581.027 nm) and (398.8 nm) transitions for various laser bandwidths of the excitation lasers. The degree of enrichment of the low-abundant isotope has been computed using the obtained isotopic selectivity values. Results obtained for scheme 1 are in good agreement with the results reported earlier. The abundance of isotope has been computed for various laser linewidths of the excitation laser and Doppler widths for scheme 2. Comparison of the two photoionization schemes for isotope-selective photoionization of the isotope has been made. The present computational method has been examined for computation of degree of enrichment for the photoionization schemes under consideration.

A theoretical study of the isotope selective excitation of 138La from 2D?,? states

Isotope ratio enhancement factors and isotopic selectivities for 138La in a natural abundance lanthanum sample as well as an enriched sample have been computed theoretically for the 5d6s2 2D32-5d26p 2F520 (593.068 nm), 5d6s2 2D52-5d26p 2F720 (593.062 nm), 5d6s2 2D32-5d26p 2D320 (550.134 nm), 5d6s2 2D52-5d26p 2D520 (545.514 nm) and 5d6s2 2D32-5d6s6p 4F320 (753.923 nm) transitions for a narrow band laser excitation in a one-photon resonant two-photon ionisation scheme considering a Lorentzian atomic profile for simulation of the excitation spectrum. Simulations have been carried out for a laser bandwidth of 50 MHz. The coherence effects are ignored in the computation as the laser bandwidth (50 MHz) is much smaller than the largest separation between the hyperfine levels (i.e. γL ⪡ ΔHFS). The results obtained for the 2D32-4F320 (753.923 nm) transition are in excellent agreement with the experimental results obtained by Young et al. using diode laser initiated resonance ionisation mass spectrometry. The isotopic selectivities and isotope ratio enhancement factors computed for the 2D52-2D520 (545.514 nm) transition are about 19% higher than the 2D32-4F320 (753.923 nm) transition used by Young et al. This increased isotopic selectivity can be used for isotope selective excitation of 138La with improved detection sensitivity. This method demonstrates its usefulness in identifying the most efficient atomic lines in order to focus experimental effort on these transitions.

A theoretical study of the isotope selective excitation of 138La from [formula omitted] states

Isotope ratio enhancement factors and isotopic selectivities for 138La in a natural abundance lanthanum sample as well as an enriched sample have been computed theoretically for the 5d6s 2 2D 3 2 - 5d 26p 2F 5 2 0 (593.068 nm), 5d6s 2 2D 5 2 - 5d 26p 2F 7 2 0 (593.062 nm), 5d6s 2 2D 3 2 - 5d 26p 2D 3 2 0 (550.134 nm), 5d6s 2 2D 5 2 - 5d 26p 2D 5 2 0 (545.514 nm) and 5d6s 2 2D 3 2 - 5d6s6p 4F 3 2 0 (753.923 nm) transitions for a narrow band laser excitation in a one-photon resonant two-photon ionisation scheme considering a Lorentzian atomic profile for simulation of the excitation spectrum. Simulations have been carried out for a laser bandwidth of 50 MHz. The coherence effects are ignored in the computation as the laser bandwidth (50 MHz) is much smaller than the largest separation between the hyperfine levels (i.e. γ L ⪡ Δ HFS). The results obtained for the 2 D 3 2 - 4 F 3 2 0 (753.923 nm) transition are in excellent agreement with the experimental results obtained by Young et al. using diode laser initiated resonance ionisation mass spectrometry. The isotopic selectivities and isotope ratio enhancement factors computed for the 2 D 5 2 - 2 D 5 2 0 (545.514 nm) transition are about 19% higher than the 2 D 3 2 - 4 F 3 2 0 (753.923 nm) transition used by Young et al. This increased isotopic selectivity can be used for isotope selective excitation of 138La with improved detection sensitivity. This method demonstrates its usefulness in identifying the most efficient atomic lines in order to focus experimental effort on these transitions.

Radiation transport within the natural isotopic mixture of rubidium vapor after isotope selective excitation

The radiation transport within the natural isotopic mixture of rubidium is studied by detecting the time dependent resonance fluorescence after isotope selective excitation of87Rb or85Rb to the 52P3/2 state by the pulsed light of a laser diode. At atomic densities of about 1012 cm−3 the mean number of photon scatterings is 10 or 25 after excitation of87Rb or85Rb, respectively. Monte Carlo simulations of multiple photon scattering are used to reproduce the time dependent fluorescence signals. Satisfactory agreement between experiments and calculations can be achieved only if a radiative transfer of excitation energy between the different isotopes is considered. The energy transfer can be explained by radiative coupling of the different isotopes due to two partially overlapping hfs components of the 780 nm resonance line. As a test of our theoretical approach fluorescence signals recorded after excitation of87Rb and85Rb under identical experimental conditions are fitted both by using the same set of two parameters.

Continuous wave field ionization laser spectroscopy

A new technique is described which allows Doppler-free, isotope-selective excitation of atoms by continuous wave laser radiation and continuous ionization of the atoms by an electric field. The atoms are excited to high Rydberg states in an electric-field-free region of a collimated atomic beam. Because the lifetimes of Rydberg atoms are long they can reach a spatially separated region of the atomic beam where they are ionized by a continuous electric field with a probability of unity. In the case of lithium we obtained a 103 times larger ion signal by field ionization of Rydberg atoms than by direct photoionization from low excited states.