Resonance Ionization Spectroscopy Research Articles

Use of a Continuous Wave Laser and Pockels Cell for Sensitive High-Resolution Collinear Resonance Ionization Spectroscopy.

New technical developments have led to a 2 orders of magnitude improvement of the resolution of the collinear resonance ionization spectroscopy (CRIS) experiment at ISOLDE, CERN, without sacrificing the high efficiency of the CRIS technique. Experimental linewidths of 20(1) MHz were obtained on radioactive beams of francium, allowing us for the first time to determine the electric quadrupole moment of the short lived [t_{1/2}=22.0(5) ms] ^{219}Fr Q_{s}=-1.21(2) eb, which would not have been possible without the advantages offered by the new method. This method relies on a continuous-wave laser and an external Pockels cell to produce narrow-band light pulses, required to reach the high resolution in two-step resonance ionization. Exotic nuclei produced at rates of a few hundred ions/s can now be studied with high resolution, allowing detailed studies of the anchor points for nuclear theories.

Resonant two-photon ionization spectroscopy of jet-cooled OsSi.

The optical spectrum of diatomic OsSi has been investigated for the first time, with transitions observed in the range from 15 212 to 18 634 cm(-1) (657-536 nm). Two electronic band systems have been identified along with a number of unclassified bands. Nine bands have been investigated at rotational resolution, allowing the ground state to be identified as X(3)Σ0(+) (-), arising from the 1σ(2)1π(4)2σ(2)3σ(2)1δ(2) configuration. The ground X(3)Σ0(+) (-) state is characterized by re = 2.1207(27) Å and ΔG1/2″ = 516.315(4) cm(-1) for the most abundant isotopologue, (192)Os(28)Si (38.63%). The A1 excited electronic state, which is thought to be primarily (3)Π1 in character, is characterized by T0 = 15 727.7(7) cm(-1), ωe = 397.0(7) cm(-1), and re = 2.236(16) Å for (192)Os(28)Si. The B1 excited electronic state is characterized by T0 = 18 468.71 cm(-1), ΔG1/2 = 324.1 cm(-1), and re = 2.1987(20) Å for (192)Os(28)Si and is thought to be primarily (1)Π1 in character. These results are placed in context through a comparison to other transition metal carbides and silicides.

Double-resonance-ionization mapping of the hyperfine structure of the stable Cu isotopes using pulsed narrowband Ti:sapphire lasers

We present two approaches to enhance the resolving power for measuring hyperfine structure constants using resonance ionization spectroscopy. The first method employs a 2D-resonance ionization spectroscopy scanning technique with pulsed, narrowband Ti:sapphire lasers (1 GHz linewidth), allowing us to resolve hyperfine components that cannot be separated using the standard 1D-scanning method across only one optical transition. In a second refinement, the resolving power is further enhanced through the use of a ring design of the laser cavity. This layout leads to a reduction of the laser linewidth from 1 GHz to below 50 MHz, resulting in experimental linewidths of about 150 MHz. Motivated by the current nuclear physics interest in radioactive Cu isotopes, the hyperfine structure of the $3{d}^{10}4s{\phantom{\rule{4pt}{0ex}}}^{2}{S}_{1/2}$ ground state and the $3{d}^{9}4s4p{\phantom{\rule{4pt}{0ex}}}^{4}{P}_{3/2}^{o}$ and $3{d}^{10}6s{\phantom{\rule{4pt}{0ex}}}^{2}{S}_{1/2}$ excited states in stable Cu isotopes were studied using both approaches. The hyperfine $A$ parameter of the $3{d}^{10}6s{\phantom{\rule{4pt}{0ex}}}^{2}{S}_{1/2}$ state was measured with values of $296(18)$ MHz for $^{63}\mathrm{Cu}$ and $307(18)$ MHz for $^{65}\mathrm{Cu}$.

Resonance-Enhanced 2-Photon Ionization Scheme for C2 through a Newly Identified Band System: 4(3)Πg-a(3)Πu.

We report the observation of a new band system of C2, namely, the 4(3)Πg-a(3)Πu system. The bands, observed by resonant 2-photon ionization spectroscopy and time-of-flight mass spectrometry, were identified through a synergy of high-level ab initio computation and double-resonance spectroscopy. Two bands are firmly identified, 1-3 and 0-2, allowing the 4(3)Πg origin to be placed at 51496.44 cm(-1). The 4(3)Πg state is characterized as having a single bond, with a vibrational frequency of about 1268 cm(-1), and an equilibrium bond length of 1.57 Å. The state is predicted to exhibit a barrier to dissociation, with a rotational constant that unusually increases with vibrational excitation up to a maximum before decreasing at higher vibrational excitation. The new band system allows us to probe the a(3)Πu state of C2 through a straightforward 1 + 1 REMPI scheme.

Selected cis- and trans-3-fluorostyrene rotamers studied by two-color resonant two-photon mass-analyzed threshold ionization spectroscopy

We applied two-color resonant two-photon ionization and mass-analyzed threshold ionization techniques to record the vibronic, photoionization efficiency, and cation spectra of the selected rotamers of 3-fluorostyrene. The adiabatic ionization energies of cis- and trans-3-fluorostyrene were determined to be 69 960±5 and 69 856±5cm−1, respectively. Cation vibrations 10a, 15, 6b, and 12 of both rotamers have been found to have frequencies of 218, 404, 452, and 971cm−1, respectively. This finding shows that the relative orientation of the vinyl group with respect to the F atom does not affect these vibrations of the 3-fluorostyrene cation. Our one-dimensional potential energy surface calculations support that the cis–trans isomerization of 3-fluorostyrene does not occur under the present experimental conditions.

Prospects for laser spectroscopy, ion chemistry and mobility measurements of superheavy elements in buffer-gas traps

Laser spectroscopic methods are reviewed which are of potential interest for the investigation of atomic and ionic level structures of superheavy elements. The latter are defined here as the trans-fermium elements with Z>100 for which no experimental atomic or ionic level structure information is known so far, and which cannot be bred in high flux nuclear power reactors via successive neutron capture. The principles of suitable laser spectroscopic methods are described, and illustrated by examples of real experiments. The addressed methods include single-ion spectroscopy in Paul traps, laser-induced fluorescence spectroscopy (LIF), radiation-detected optical pumping (RADOP), radioactive decay-detected resonance ionization spectroscopy (RADRIS), and ion-guide-detected resonance ionization spectroscopy (IGRIS). With the exception of the first all take advantage of a storage of the ions or atoms in so-called buffer-gas traps.The developed experimental methods can, in principle, also be employed for studying ion-chemical reactions with gas admixtures like O2 as well as for performing ion mobility measurements. Both provide complementary information on the electronic structure of superheavy ions. First attempts on this road of research are reviewed as well.

The elusive S2 state, the S1/S2 splitting, and the excimer states of the benzene dimer

We observe the weak S0 → S2 transitions of the T-shaped benzene dimers (Bz)2 and (Bz-d6)2 about 250 cm(-1) and 220 cm(-1) above their respective S0 → S1 electronic origins using two-color resonant two-photon ionization spectroscopy. Spin-component scaled (SCS) second-order approximate coupled-cluster (CC2) calculations predict that for the tipped T-shaped geometry, the S0 → S2 electronic oscillator strength fel(S2) is ∼10 times smaller than fel(S1) and the S2 state lies ∼240 cm(-1) above S1, in excellent agreement with experiment. The S0 → S1 (ππ(∗)) transition is mainly localized on the "stem" benzene, with a minor stem → cap charge-transfer contribution; the S0 → S2 transition is mainly localized on the "cap" benzene. The orbitals, electronic oscillator strengths fel(S1) and fel(S2), and transition frequencies depend strongly on the tipping angle ω between the two Bz moieties. The SCS-CC2 calculated S1 and S2 excitation energies at different T-shaped, stacked-parallel and parallel-displaced stationary points of the (Bz)2 ground-state surface allow to construct approximate S1 and S2 potential energy surfaces and reveal their relation to the "excimer" states at the stacked-parallel geometry. The fel(S1) and fel(S2) transition dipole moments at the C2v-symmetric T-shape, parallel-displaced and stacked-parallel geometries are either zero or ∼10 times smaller than at the tipped T-shaped geometry. This unusual property of the S0 → S1 and S0 → S2 transition-dipole moment surfaces of (Bz)2 restricts its observation by electronic spectroscopy to the tipped and tilted T-shaped geometries; the other ground-state geometries are impossible or extremely difficult to observe. The S0 → S1/S2 spectra of (Bz)2 are compared to those of imidazole ⋅ (Bz)2, which has a rigid triangular structure with a tilted (Bz)2 subunit. The S0 → S1/ S2 transitions of imidazole-(benzene)2 lie at similar energies as those of (Bz)2, confirming our assignment of the (Bz)2 S0 → S2 transition.

Vibronic spectroscopy of a nitrile/isonitrile isoelectronic pair: para-diisocyanobenzene and para-isocyanobenzonitrile.

The ultraviolet spectroscopy of isoelectronic pair para-diisocyanobenzene (pDIB) and para-isocyanobenzonitrile (pIBN) has been studied under gas-phase, jet-cooled conditions. These molecules complete a sequence of mono and disubstituted nitrile/isonitrile benzene derivatives, enabling a comparison of the electronic effects of such substitution. Utilizing laser-induced fluorescence (LIF) and resonant two-photon ionization (R2PI) spectroscopy, the S0-S1 electronic origins of pDIB and pIBN have been identified at 35,566 and 35,443 cm(-1), respectively. In pDIB, the S0-S1 origin is very weak, with b(3g) fundamentals induced by vibronic coupling to the S2 state dominating the spectrum at 501 cm(-1) (ν17, isocyano bend) and 650 cm(-1) (ν16, ring distortion). The spectrum extends over 5000 cm(-1), remaining sharp and relatively uncongested over much of this range. Dispersed fluorescence (DFL) spectra confirm the dominating role played by vibronic coupling and identify Franck-Condon active ring modes built off the vibronically-induced bands. In pDIB, the S2 state has been tentatively observed at about 6100 cm(-1) above the S0-S1 origin. In pIBN, the S0-S1 origin is considerably stronger, but vibronic coupling still plays an important role, involving fundamentals of b2 symmetry. The bending mode of the nitrile group dominates the vibronically-induced activity. Calculations carried out at the TD-DFT B3LYP/6-31+G(d) level of theory account for the extremely weak S0-S1 oscillator strength of pDIB and the larger intensity of the S0-S1 origins of pIBN and pDCB (para-dicyanobenzene) as nitrile groups are substituted for isonitrile groups. In pDIB, a nearly perfect cancellation of transition dipoles occurs due to two one-electron transitions that contribute nearly equally to the S0-S1 transition. The spectra of both molecules show no clear evidence of charge-transfer interactions that play such an important role in some cyanobenzene derivatives.

Resonant two-photon ionization spectroscopy of Al atoms and dimers solvated in helium nanodroplets.

Resonant two-photon ionization (R2PI) spectroscopy has been applied to investigate the solvation of Al atoms in helium droplets. The R2PI spectra reveal vibrational progressions that can be attributed to Al-He(n) vibrations. It is found that small helium droplets have very little chance to pick up an aluminum atom after collision. However, the pick-up probability increases with the size of the helium droplets. The absorption band that is measured by monitoring the ions on the mass of the Al dimer is found to be very little shifted with respect to the Al monomer band (∼400 cm(-1)). However, using the same laser wavelength, we were unable to detect any Al(n) photoion with n larger than two.

In-Source Laser Spectroscopy with the Laser Ion Source and Trap: First Direct Study of the Ground-State Properties ofPo217,219

A Laser Ion Source and Trap (LIST) for a thick-target, isotope-separation on-line facility has been implemented at CERN ISOLDE for the production of pure, laser-ionized, radioactive ion beams. It offers two modes of operation, either as an ion guide, which performs similarly to the standard ISOLDE resonance ionization laser ion source (RILIS), or as a more selective ion source, where surface-ionized ions from the hot ion-source cavity are repelled by an electrode, while laser ionization is done within a radiofrequency quadrupole ion guide. The first physics application of the LIST enables the suppression of francium contamination in ion beams of neutron-rich polonium isotopes at ISOLDE by more than 1000 with a reduction in laser-ionization efficiency of only 20. Resonance ionization spectroscopy is performed directly inside the LIST device, allowing the study of the hyperfine structure and isotope shift of 217Po for the first time. Nuclear decay spectroscopy of 219Po is performed for the first time, revealing its half-life, ?- to-?-decay branching ratio, and ?-particle energy. This experiment demonstrates the applicability of the LIST at radioactive ion-beam facilities for the production and study of pure beams of exotic isotopes.

Polarization-based isotope-selective two-color photoionization of atomic samarium using broadband lasers

An isotope separation method based on polarization selection rules is applied to atomic samarium by using two-color resonance ionization spectroscopy with broadband lasers. In this method, odd isotopes with nonzero nuclear spin are selectively excited, while even isotopes with zero nuclear spin are prohibited from excitation using two parallel linearly polarized lasers. We have identified a two-color excitation scheme 0 cm−1 (J = 0) → 15650.5 cm−1 (J = 1) → 33116.8 cm−1 (J = 1) → Sm+ for selective excitation of the odd isotopes of Sm I. Using this scheme, selective excitation of odd isotopes of Sm I (147Sm and 149Sm) with an isotopic selectivity better than 40 has been demonstrated. In addition, the effect of different polarization states of the excitation lasers and relative polarization angle between them on the selectivity of odd isotopes has also been studied. The dependence of the even mass isotope signal on the relative polarization angle followed sin2θ, which is in excellent agreement with theoretical predictions.

Optical spectrum of Si3C, and a re-analysis of the [formula omitted] transition

The electronic spectrum of Si3C has been measured in the 710–250nm region by resonant two-color two-photon ionization spectroscopy. In addition to the strong C∼1B1←X∼1A1 transition near 400nm, evidence is found for two weaker features: a broad, largely structureless band near 625nm, which is assigned to A∼1B1←X∼1A1, and a band near 333nm with partially-resolved vibronic structure, which we assign to E∼1B1←X∼1A1. Assignment of the transitions and analysis of their vibrational structure is supported by equation-of-motion coupled cluster calculations. Re-measurement and re-analysis of the C∼←X∼ transition at higher spectral resolution and signal-to-noise ratio than that reported previously (Stanton et al., 2005) allows essentially all of the vibronic features to be assigned with confidence.

Barium Tagging from nEXO Using Resonance Ionization Spectroscopy

nEXO is a 5-ton liquid enriched-xenon time projection chamber (TPC) to search for neutrinoless double-beta decay, designed to have the sensitivity to completely probe the inverted mass hierarchy of Majorana neutrinos. The detector will accommodate—as a background reduction technique—a system to recover and identify the barium decay product. This upgrade will allow a background-free measurement of neutrinoless double-beta decay and increase the half-life sensitivity of the experiment by at least one order of magnitude. Ongoing research and development includes a system to test barium extraction from liquid xenon using surface adsorption and Resonance Ionization Spectroscopy (RIS).

Experimental and theoretical study on the excited-state dynamics of ortho-, meta-, and para-methoxy methylcinnamate.

The S1 state dynamics of methoxy methylcinnamate (MMC) has been investigated under supersonic jet-cooled conditions. The vibrationally resolved S1-S0 absorption spectrum was recorded by laser induced fluorescence and mass-resolved resonant two-photon ionization spectroscopy and separated into conformers by UV-UV hole-burning (UV-UV HB) spectroscopy. The S1 lifetime measurements revealed different dynamics of para-methoxy methylcinnamate from ortho-methoxy methylcinnamate and meta-methoxy methylcinnamate (hereafter, abbreviated as p-, o-, and m-MMCs, respectively). The lifetimes of o-MMC and m-MMC are on the nanosecond time scale and exhibit little tendency of excess energy dependence. On the other hand, p-MMC decays much faster and its lifetime is conformer and excess energy dependent. In addition, the p-MMC-H2O complex was studied to explore the effect of hydration on the S1 state dynamics of p-MMC, and it was found that the hydration significantly accelerates the nonradiative decay. Quantum chemical calculation was employed to search the major decay route from S1(ππ(∗)) for three MMCs and p-MMC-H2O in terms of (i) trans → cis isomerization and (ii) internal conversion to the (1)nπ(∗) state. In o-MMC and m-MMC, the large energy barrier is created for the nonradiative decay along (i) the double-bond twisting coordinate (∼1000 cm(-1)) in S1 as well as (ii) the linear interpolating internal coordinate (∼1000 cm(-1)) from S1 to (1)nπ(∗) states. The calculation on p-MMC decay dynamics suggests that both (i) and (ii) are available due to small energy barrier, i.e., 160 cm(-1) by the double-bond twisting and 390 cm(-1) by the potential energy crossing. The hydration of p-MMC raises the energy barrier of the IC route to the S1/(1)nπ(∗) conical intersection, convincing that the direct isomerization is more likely to occur.

Excitonic Splitting, Delocalization, and Vibronic Quenching in the Benzonitrile Dimer

The excitonic S1/S2 state splitting and the localization/delocalization of the S1 and S2 electronic states are investigated in the benzonitrile dimer (BN)2 and its (13)C and d5 isotopomers by mass-resolved two-color resonant two-photon ionization spectroscopy in a supersonic jet, complemented by calculations. The doubly hydrogen-bonded (BN-h5)2 and (BN-d5)2 dimers are C2h symmetric with equivalent BN moieties. Only the S0 → S2 electronic origin is observed, while the S0 → S1 excitonic component is electric-dipole forbidden. A single (12)C/(13)C or 5-fold h5/d5 isotopic substitution reduce the dimer symmetry to Cs, so that the heteroisotopic dimers (BN)2-(h5 – h5(13)C), (BN)2-(h5 – d5), and (BN)2-(h5 – h5(13)C) exhibit both S0 → S1 and S0 → S2 origins. Isotope-dependent contributions Δiso to the excitonic splittings arise from the changes of the BN monomer zero-point vibrational energies; these range from Δiso((12)C/(13)C) = 3.3 cm(–1) to Δiso(h5/d5) = 155.6 cm(–)1. The analysis of the experimental S1/S2 splittings of six different isotopomeric dimers yields the S1/S2 exciton splitting Δexc = 2.1 ± 0.1 cm(–1). Since Δiso(h5/d5) ≫ Δexc and Δiso((12)C/(13)C) > Δexc, complete and near-complete exciton localization occurs upon (12)C/(13)C and h5/d5 substitutions, respectively, as diagnosed by the relative S0 → S1 and S0 → S2 origin band intensities. The S1/S2 electronic energy gap of (BN)2 calculated by the spin-component scaled approximate second-order coupled-cluster (SCS-CC2) method is Δel(calc) = 10 cm(–1). This electronic splitting is reduced by the vibronic quenching factor Γ. The vibronically quenched exciton splitting Δel(calc)·Γ = Δvibron(calc) = 2.13 cm(–1) is in excellent agreement with the observed splitting Δexc = 2.1 cm(–1). The excitonic splittings can be converted to semiclassical exciton hopping times; the shortest hopping time is 8 ps for the homodimer (BN-h5)2, the longest is 600 ps for the (BN)2(h5 – d5) heterodimer.

An apparatus to manipulate and identify individual Ba ions from bulk liquid Xe.

We describe a system to transport and identify barium ions produced in liquid xenon, as part of R&D towards the second phase of a double beta decay experiment, nEXO. The goal is to identify the Ba ion resulting from an extremely rare nuclear decay of the isotope (136)Xe, hence providing a confirmation of the occurrence of the decay. This is achieved through Resonance Ionization Spectroscopy (RIS). In the test setup described here, Ba ions can be produced in liquid xenon or vacuum and collected on a clean substrate. This substrate is then removed to an analysis chamber under vacuum, where laser-induced thermal desorption and RIS are used with time-of-flight mass spectroscopy for positive identification of the barium decay product.

Laser spectroscopy of francium isotopes at the borders of the region of reflection asymmetry

The magnetic dipole moments and changes in mean-square charge radii of the neutron-rich $^{218m,219,229,231}\text{Fr}$ isotopes were measured with the newly-installed Collinear Resonance Ionization Spectroscopy (CRIS) beam line at ISOLDE, CERN, probing the $7s~^{2}S_{1/2}$ to $8p~^{2}P_{3/2}$ atomic transition. The $\delta\langle r^{2}\rangle^{A,221}$ values for $^{218m,219}\text{Fr}$ and $^{229,231}\text{Fr}$ follow the observed increasing slope of the charge radii beyond $N~=~126$. The charge radii odd-even staggering in this neutron-rich region is discussed, showing that $^{220}\text{Fr}$ has a weakly inverted odd-even staggering while $^{228}\text{Fr}$ has normal staggering. This suggests that both isotopes reside at the borders of a region of inverted staggering, which has been associated with reflection-asymmetric shapes. The $g(^{219}\text{Fr}) = +0.69(1)$ value supports a $\pi 1h_{9/2}$ shell model configuration for the ground state. The $g(^{229,231}\text{Fr})$ values support the tentative $I^{\pi}(^{229,231}\text{Fr}) = (1/2^{+})$ spin, and point to a $\pi s_{1/2}^{-1}$ intruder ground state configuration.

Mimicking anesthetic-receptor interactions in jets: the propofol-isopropanol cluster.

The interaction of the general anesthetic propofol with an individual residue of threonine in the membrane receptors has been modeled in the gas phase by examining the adduct of propofol with the isopropanol side-chain. We determined the structural preferences of the cluster using a combination of mass-resolved laser spectroscopy and quantum mechanical calculations. The first electronic transition of propofol-isopropanol was recorded with vibrational resolution using resonant two-photon ionization (R2PI) and ion dip IR spectroscopy. The spectra obtained were compared with density-functional calculations (DFT) using the M06-2X functional in order to obtain the cluster's structure. Three isomers have been detected. The results suggest that propofol acts as a Brønsted acid, donating a proton to the isopropanol molecule in a conformation that resembles that of propofol-water, but displaced towards the aromatic ring, due to the interaction with the aliphatic side of isopropanol. The higher affinity of propofol for isopropanol compared to water may correlate with the biological role of propofol at the protein binding site. On the other hand, propofol shows a similar affinity for isopropanol and phenol, which could explain the mobility that propofol experiences inside the GABAA cavity.

New even-parity high-lying levels of Sm I and measurement of isotope shifts by two-color resonance ionization mass spectrometry

In this work, we investigate the even-parity high-lying levels of Sm I in the energy region 33136–33960cm−1 by performing two-color three-photon resonance ionization spectroscopy in an atomic beam coupled to a time-of-flight mass spectrometer using two tunable pulsed dye lasers. We observe twenty-one new and confirm eight previously reported even-parity energy levels of Sm I in this spectral region. Absolute energies of these levels are determined with an accuracy of ±0.3cm−1. Using electric dipole selection rule, total angular momentum (J-value) of the most newly observed levels is assigned uniquely. Further, employing two-color three-step resonance ionization mass spectrometry, we measure the isotope shift between 154Sm and 144Sm of sixteen high-lying levels with a moderate accuracy of ±30mK.

RILIS applications at CERN/ISOLDE

Since 2011 the ISOLDE Resonance Ionization Laser Ion Source (RILIS) has comprised a dual system of three dye and three Ti:sapphire lasers capable of gap-free wavelength tuning from 210 to 950 nm. Temporal synchronization of the lasers has enabled several operating modes to be established which make optimal use of the complementary characteristics of each laser type. This flexibility of the system has presented several opportunities for additional atomic spectroscopy studies and ionization scheme development, whilst also enabling an increase in the number of operating hours for standard ion beam production. The extended capabilities of the dual-RILIS system are exemplified by the recent operational highlights. These include on-line ionization scheme development (At, Ca), measurements of ionization energies (At, Po), in-source resonance ionization spectroscopy of exotic isotopes (At, Au, Po), and the provision of a fibre-coupled narrow-linewidth Ti:sapphire beam for the on-line commissioning of the CRIS experiment (Fr).