Resonance-enhanced Multiphoton Ionization Research Articles

Ion core switching during photodissociation dynamics via the Rydberg states of XeAr

The heterodimer, XeAr, is a classic example of a weakly bound van der Waals molecule, which has a variety of accessible bound excited states that exhibit complex interactions. In this study, XeAr has been investigated in the 77,500–81,500 cm−1 region using a combination of Resonance Enhanced Multi-Photon Ionization (REMPI) spectroscopy and Velocity Map Imaging (VMI). By monitoring REMPI and photodissociative product channels across the spectrum, several novel excited states, product channels, excited state symmetries and lifetimes, as well as highly localized perturbations were observed and characterized, including the first VMI study of Ar* dissociating from XeAr Rydberg states accessed by two-photon excitation. In this work we have analyzed 38 vibronic bands representing nine different electronic transitions, and we provide new assignments for two 0+←0+ electronic transitions dissociating to the Xe* 5d [3/2]02 (ca. 80,323 cm−1) and Xe* 5d [7/2]03 (ca. 80,970 cm−1) limits. Several new predissociation product channels were identified at the two- and three-photon levels, including production of Xe* 5p[5/2]3, Xe* 6s'[1/2]o1, Xe* 6p[1/2]1, Ar* 4p[1/2]0, Ar* 4p'[3/2]1, Ar* 4p'[1/2]1, and Ar* 4p[5/2]3. Using the multidimensional analysis offered by VMI, we explore interesting photophysics whereby a resonant state that is reached after absorbing two photons can predissociate to yield Xe*, but which also can absorb a 3rd photon, yielding super-excited Ar*Xe that predissociates to Ar* limits. The ground state dissociation energy for XeAr was determined to be D0 = 114.4 ± 2.7 cm−1, in excellent agreement with previous measurements.

N-H and N-C Bond Dissociation Pathways in Ultraviolet Photodissociation of Dimethylamine.

We investigated the interlinked N-H and N-C photochemistry of primary and secondary amines via the state-resolved detection of vibrationally excited CH3 product and H atom product by 200-235 nm dimethylamine photodissociation using resonance-enhanced multiphoton ionization (REMPI) and velocity map imaging (VMI) techniques. The out-of-plane bending (ν2) vibrationally excited CH3 showed a bimodal translational energy distribution that became unimodal with a near-zero product yield at longer photolysis wavelengths (λphotolysis). In contrast, a unimodal distribution was observed for the C-H stretching (νCH) vibrationally excited CH3 products with an almost constant product yield in the examined λphotolysis region. We ascribed the state-specific energy releases of the CH3 products to two reaction pathways based on calculations of the potential energy surface (PES): the direct N-CH3 dissociation pathway and the indirect N-CH3 dissociation pathway via the N-H bond conical intersection. Meanwhile, the H atom product showed a bimodal energy distribution similar to the ammonia photodissociation model, with an excited-state counterproduct channel that became accessible at a shorter λphotolysis. These results suggest that the N-H and N-C bond dissociations are connected, and these dissociations cause different photochemistry between primary/secondary amines and tertiary amines.

Significantly Improved Detection of Molecular Oxygen by Two-Color Resonance-Enhanced Multiphoton Ionization.

We report a new spectroscopic detection scheme for molecular oxygen that achieves roughly two orders of magnitude higher sensitivity for fully rotationally resolved spectra than the current state of the art. Two-color (2 + 1') resonance-enhanced multiphoton ionization (REMPI) via the 3d Rydberg complex yields state-selective spectra with signal comparable to the intense but diffuse C 3sσ 3Πg ← X 3Σg- (2 + 1) REMPI bands without significant saturation or broadening. The resulting increase in sensitivity permitted observation of the very weak 3dπ 1Δ2 ← X 3Σg- transitions and is independent of the intermediate state. This advance in ionization efficiency and quantum state-selective sensitivity for O2 promises to aid physical and chemical studies across a wide variety of fields.

Long-lived nitric oxide molecular tagging velocimetry with 1 + 1 REMPI.

The successful demonstration of long-lived nitric oxide (NO) fluorescence for molecular tagging velocimetry (MTV) measurements is described in this Letter. Using 1 + 1 resonance-enhanced multiphoton ionization (REMPI) of NO at a wavelength near 226 nm, targeting the overlapping Q1(7) and Q21(7) lines of the A-X (0, 0) electronic system, the lifetime of the NO MTV signal was observed to be approximately 8.6 µs within a 100-Torr cell containing 2% NO in nitrogen. This is in stark contrast to the commonly reported single photon NO fluorescence, which has a much shorter calculated lifetime of approximately 43 ns at this pressure and NO volume fraction. While the shorter lifetime fluorescence can be useful for molecular tagging velocimetry with single laser excitation within very high-speed flows at some thermodynamic conditions, the longer lived fluorescence shows the potential for an order of magnitude more accurate and precise velocimetry, particularly within lower speed regions of hypersonic flow fields such as wakes and boundary layers. The physical mechanism responsible for the generation of this long-lived signal is detailed. Furthermore, the effectiveness of this technique is showcased in a high-speed jet flow, where it is employed for precise flow velocity measurements.

The Electronic Structures of Azaphenanthrenes and Their Dimers.

Insertion of a nitrogen atom modifies the electronic structures and photochemistry of polycyclic aromatic hydrocarbons by introducing nπ* states into the molecules. To better understand the electronic structures of isolated polycyclic aromatic nitrogen-containing hydrocarbons (PANHs) and their dimers as well as the influence of the position of the nitrogen atom in the molecule, we investigate three different azaphenanthrenes, benzo[f]quinoline, benzo[h]quinoline, and phenanthridine, in a joint experimental and computational study. Experimentally, resonance-enhanced multiphoton ionization (REMPI) spectroscopy is applied to characterize the excited electronic states. The REMPI spectra of the azaphenanthrene monomers have a rather similar appearance, with origins between 3.645 and 3.670 eV for the 1ππ* ← S0 transition. In contrast to the phenanthrene parent, 2ππ* ← S0 is broad and unstructured even at the band origin. The experiments are accompanied by density functional theory computation, and vibrationally resolved spectra are simulated using a time-independent approach. The differences between phenanthrene and the azaphenanthrenes are assigned to perturbations due to the low-lying 1(nπ*) state, which accelerates nonradiative deactivation. For the dimers, it is found that two π-stacked isomers with two electronic transitions each contribute to the electronic spectrum, leading to overlapping bands that are difficult to assign.

Spectroscopic Study of a New Electronic Band System 33Δg-a3Πu of C2.

As one of the most important diatomic molecules in the universe, the spectroscopic characterizations of C2 have attracted wide attention in various fields, such as interstellar chemistry, planetary atmospheric chemistry, and combustion. In recent years, a systematic spectroscopic study of C2 in the vacuum ultraviolet (VUV) region has been carried out in our laboratory by using the (1VUV+1'UV) resonance-enhanced multiphoton ionization method based on the combination of a tunable VUV laser source and a time-of-flight mass spectrometer. Two new electronic transition band systems have been reported, following the pioneering work of Herzberg and co-workers in 1969. In the current study, a total of 18 vibronic transition bands of C2 from the lower a3Πu state are experimentally observed in the VUV photon energy range 72000-81000 cm-1, and 6 new upper vibronic levels of 3Δg symmetry are identified, which are assigned as the v' = 0-5 vibrational levels of the 33Δg state of C2. The term energy Te of the 33Δg state is determined to be in the range of 78425-78475 cm-1 (9.724-9.730 eV) with respect to the ground X1Σg+ state, and the molecular constants such as vibrational and rotational constants are also determined, which are in reasonable agreement with those predicted by high-level ab initio theoretical calculations. Irregular vibrational energy level spacings in the 33Δg state are observed, which is tentatively attributed to the strong perturbations between the 33Δg and 23Δg states, as previously predicted by theory.

Mass spectrometric analysis of unprecedented high levels of carbonaceous aerosol particles long-range transported from wildfires in the Siberian Arctic

Abstract. Wildfires in Siberia generate large amounts of aerosols, which may be transported over long distances and pose a threat to the sensitive ecosystem of the Arctic. Particulate matter (PM) of aged wildfire plumes originating from Yakutia in August 2021 was collected in Nadym and on Bely Island (both in northwestern Siberia). An advanced analysis of the chemical composition of aerosol particles was conducted through a multi-wavelength thermal–optical carbon analyzer (TOCA) coupled to resonance-enhanced multiphoton ionization time-of-flight mass spectrometry (REMPI-TOFMS) as well as through ultra-high-resolution Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). In Nadym, concentrations of organic carbon (OC) and elemental carbon (EC) peaked at 100 and 40 µg m−3, respectively, associated with Angström absorption exponents for 405 and 808 nm (AAE405/808) between 1.5 and 3.3. The weekly average on Bely Island peaked at 8.9 µg m−3 of OC and 0.3 µg m−3 of EC and AAE405/808 close to unity. In particular, ambient aerosol in Nadym had a distinct biomass burning profile with pyrolysis products from carbohydrates, such as cellulose and hemicellulose, as well as lignin and resinoic acids. However, temporarily higher concentrations of five- and six-ring polycyclic aromatic hydrocarbons (PAHs), different from the PAH signature of biomass burning, suggest a contribution of regional gas flaring. FT-ICR MS with electrospray ionization (ESI) revealed a complex mixture of highly functionalized compounds, containing up to 20 oxygen atoms, as well as nitrogen- and sulfur-containing moieties. Concentrations of biomass burning markers on Bely Island were substantially lower than in Nadym, flanked by the appearance of unique compounds with higher oxygen content, higher molecular weight, and lower aromaticity. Back-trajectory analysis and satellite-derived aerosol optical depth suggested long-range transport of aerosol from the center of a Yakutian wildfire plume to Nadym and from the plume periphery to Bely Island. Owing to lower aerosol concentrations in the plume periphery than in its center, it is demonstrated how dilution affects the chemical plume composition during atmospheric aging.

Attosecond time delay during resonance-enhanced multiphoton ionization of Ar in strong laser fields

Attosecond time delay during resonance-enhanced multiphoton ionization of Ar in strong laser fields

Capturing the generation and structural transformations of molecular ions.

Molecular ions are ubiquitous and play pivotal roles1-3 in many reactions, particularly in the context of atmospheric and interstellar chemistry4-6. However, their structures and conformational transitions7,8, particularly in the gas phase, are less explored than those of neutral molecules owing to experimental difficulties. A case in point is the halonium ions9-11, whose highly reactive nature and ring strain make them short-lived intermediates that are readily attacked even by weak nucleophiles and thus challenging to isolate or capture before they undergo further reaction. Here we show that mega-electronvolt ultrafast electron diffraction (MeV-UED)12-14, used in conjunction with resonance-enhanced multiphoton ionization, can monitor the formation of 1,3-dibromopropane (DBP) cations and their subsequent structural dynamics forming a halonium ion. We find that the DBP+ cation remains for a substantial duration of 3.6 ps in aptly named 'dark states' that are structurally indistinguishable from the DBP electronic ground state. The structural data, supported by surface-hopping simulations15 and ab initio calculations16, reveal that the cation subsequently decays to iso-DBP+, an unusual intermediate with a four-membered ring containing a loosely bound17,18 bromine atom, and eventually loses the bromine atom and forms a bromonium ion with a three-membered-ring structure19. We anticipate that the approach used here can also be applied to examine the structural dynamics of other molecular ions and thereby deepen our understanding of ion chemistry.

Selective ionization of marker molecules in fuels by laser-based ion mobility spectrometry (LIMS).

Careful quality control of complex matrices such as fuels and food is necessary due to the prevalence of counterfeit and pirated goods in global trade. The addition of taggants (indicator substances) to products or their packaging helps to ensure traceability. In order to prevent the mixing of different liquid products, such as different taxed fuels, invisible labelling (marker) can be used to detect illegal activities. This study investigates the qualitative and quantitative analysis of markers in complex fuel matrices using Resonance-Enhanced Multiphoton Ionisation (REMPI) Ion Mobility Spectrometry (IMS). The potential of REMPI as a selective ionisation technique for the detection of markers is highlighted, particularly with respect to minimizing matrix background and the possibility of detection without chromatographic pre-separation. Finding a suitable marker-wavelength combination that provides a suitable marker-to-matrix ratio allows selective ionization of markers while minimising matrix background. Matrix analysis shows that higher excitation wavelengths result in reduced matrix signals, with the low intensities observed at 355 nm for diesel and petrol matrices. Several candidate markers are evaluated based on the criteria of intense signal at 355 nm and non-leachability for the low tax labelling. The analytical performance of selected markers is evaluated, with a focus on the charge transfer reaction (CTR) between markers and matrix components. Our findings demonstrate the potential of REMPI-IMS for marker analysis in fuels without the need for chromatographic pre-separation, providing a promising approach for detecting illegal or fraudulent activities in the supply chain.

The tautomer-specific excited state dynamics of 2,6-diaminopurine using resonance-enhanced multiphoton ionization and quantum chemical calculations.

2,6-Diaminopurine (2,6-dAP) is an alternative nucleobase that potentially played a role in prebiotic chemistry. We studied its excited state dynamics in the gas phase by REMPI, IR-UV hole burning, and ps pump-probe spectroscopy and performed quantum chemical calculations at the SCS-ADC(2) level of theory to interpret the experimental results. We found the 9H tautomer to have a small barrier to ultrafast relaxation via puckering of its 6-membered ring. The 7H tautomer has a larger barrier to reach a conical intersection and also has a sizable triplet yield. These results are discussed relative to other purines, for which 9H tautomerization appears to be more photostable than 7H and homosubstituted purines appear to be less photostable than heterosubstituted or singly substituted purines.

Roaming in the Unimolecular Decay of syn-Methyl-Substituted Criegee Intermediates.

Alkene ozonolysis generates transient carbonyl oxide species, known as Criegee intermediates, which are a significant nonphotolytic source of OH radicals in the troposphere. This study demonstrates that unimolecular decay of syn-methyl-substituted Criegee intermediates proceeds via 1,4 H atom transfer to vinyl hydroperoxides, resulting in OH fission to O-O products or, alternatively, OH roaming to hydroxycarbonyl products. Newly generated Criegee intermediates are shown to yield hydroxycarbonyls with sufficient internal excitation to dissociate via C-C fission to acyl and hydroxymethyl (CH2OH) radicals. The stabilized Criegee intermediates and unimolecular products are rapidly cooled in a pulsed supersonic expansion for photoionization detection with time-of-flight mass spectrometry. CH2OH products are identified by 2 + 1 resonance-enhanced multiphoton ionization via the 3pz Rydberg state upon unimolecular decay of CH3CHOO, (CH3)2COO, (CH3)(CH3CH2)COO, and (CH3)(CH2═CH)COO (methyl vinyl ketone oxide). The stabilized Criegee intermediates are separately detected using 10.5 eV photoionization. This study provides the first experimental evidence of roaming in the unimolecular decay of isoprene-derived methyl vinyl ketone oxide and extends earlier studies that reported stabilized hydroxycarbonyl products.

Laser-Pulse-Length Effects in Ultrafast Laser Desorption.

Shortening the laser pulse length opens up new opportunities for laser desorption (LD) of molecules, with benefits for mass spectrometry (MS) sampling and ionization. The capability to ablate any material without the need for an absorbing matrix and the decrease of thermal damage and molecular fragmentation has promoted various applications with very different parameters and postionization techniques. However, the key issues of the optimum laser pulse length and intensity to achieve efficient and gentle desorption of molecules for postionization in MS are not resolved, although these parameters determine the costs and complexity of the required laser system. Here, we address this research gap with a systematic study on the effect of the pulse length on the LD of molecules. Keeping all other optical and ionization parameters constant, we directly compared the pulses in the femtosecond, picosecond, and nanosecond range with respect to LD-induced fragmentation and desorption efficiency. To represent real-world applications, we investigated the LD of over-the-counter medicaments naproxen and ibuprofen directly from tablets as well as the LD of retene and ship emission aerosols from a quartz filter. With our study design, we excluded interfering effects on fragmentation and LD efficiency from, for example, collisional cooling or postionization by performing the experiments in vacuum with resonance-enhanced multiphoton ionization as the postionization technique. Regarding LD-induced fragmentation, we already found benefits for the picosecond pulses. However, the efficiency of LD was found to continuously increase with decreasing pulse length, pointing to the application potential of ultrashort pulses in trace analytics. Because many interfering effects beyond the LD pulse length could be excluded in the experiment, our results may be directly transferable to the LD applied in other techniques.

Low temperature reaction kinetics inside an extended Laval nozzle: REMPI characterization and detection by broadband rotational spectroscopy.

Chirped-Pulse Fourier-Transform millimeter wave (CP-FTmmW) spectroscopy is a powerful method that enables detection of quantum state specific reactants and products in mixtures. We have successfully coupled this technique with a pulsed uniform Laval flow system to study photodissociation and reactions at low temperature, which we refer to as CPUF ("Chirped-Pulse/Uniform flow"). Detection by CPUF requires monitoring the free induction decay (FID) of the rotational coherence. However, the high collision frequency in high-density uniform supersonic flows can interfere with the FID and attenuate the signal. One way to overcome this is to sample the flow, but this can cause interference from shocks in the sampling region. This led us to develop an extended Laval nozzle that creates a uniform flow within the nozzle itself, after which the gas undergoes a shock-free secondary expansion to cold, low pressure conditions ideal for CP-FTmmW detection. Impact pressure measurements, commonly used to characterize Laval flows, cannot be used to monitor the flow within the nozzle. Therefore, we implemented a REMPI (resonance-enhanced multiphoton ionization) detection scheme that allows the interrogation of the conditions of the flow directly inside the extended nozzle, confirming the fluid dynamics simulations of the flow environment. We describe the development of the new 20K extended flow, along with its characterization using REMPI and computational fluid dynamics. Finally, we demonstrate its application to the first low temperature measurement of the reaction kinetics of HCO with O2 and obtain a rate coefficient at 20K of 6.66 ± 0.47 × 10-11cm3 molec-1s-1.

Molecular beam laser spectroscopic studies of the photoactive properties of resveratrol

Resonance Enhanced MultiPhoton Ionization spectroscopic techniques coupled with laser desorption and supersonic cooling have been employed to elucidate the photoactive properties of resveratrol. The observed excitation spectra give evidence for an internal-energy dependent trans–cis isomerisation pathway in the electronically excited state, while pump-probe studies show dynamics that are in line with what is known for the parent compound, trans-stilbene. Similar studies have been performed on a derivative of resveratrol with methoxy instead of hydroxy groups, a compound aimed to reduce previously observed photodegradation pathways of resveratrol. Time-resolved studies of the latter compound under solution conditions have given further insight into its excited-state dynamics and support the isolated-molecule conclusions on the topology of the potential energy surface of the electronically excited state. Spectroscopic studies under prolonged irradiation conditions show that both compounds suffer from photodegradation, although in the alkylated compound other pathways appear to be involved than in resveratrol.Graphical abstract

MALDI-2 Mass Spectrometry for Synthetic Polymer Analysis.

Synthetic polymers are ubiquitous in daily life, and their properties offer diverse benefits in numerous applications. However, synthetic polymers also present an increasing environmental burden through their improper disposal and subsequent degradation into secondary micro- and nanoparticles (MNPs). These MNPs accumulate in soil and water environments and can ultimately end up in the food chain, resulting in potential health risks. Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) has the potential to study localized biological or toxicological changes in organisms exposed to MNPs. Here, we investigate whether MALDI-2 postionization can provide a sensitivity enhancement in polymer analysis that could contribute to the study of MNPs. We evaluated the effect of MALDI-2 by comparing MALDI and MALDI-2 ion yields from polyethyleneglycol (PEG), polypropylene glycol (PPG), polytetrahydrofuran (PTHF), nylon-6, and polystyrene (PS). MALDI-2 caused a signal enhancement of the protonated species for PEG, PPG, PTHF, and nylon-6. PS, by contrast, preferentially formed radical ions, which we attribute to direct resonance-enhanced multiphoton ionization (REMPI). REMPI of PS led to an improvement in sensitivity by several orders of magnitude, even without cationizing salts. The improved sensitivity demonstrated by MALDI-2 for all polymers tested highlights its potential for studying the distribution of certain classes of polymers in biological systems.

Excited-state dynamics of deuterated indigo

Indigo, a rich blue dye, is an incredibly photostable molecule that has survived in ancient art for centuries. It is also unique in that it can undergo both an excited-state hydrogen and proton transfer on the picosecond timescale followed by a ground-state back transfer. Previously, we performed gas phase excited-state lifetime studies on indigo to study these processes in a solvent-free environment, combined with excited-state calculations. We found two decay pathways, a fast sub-nanosecond decay and a slow decay on the order of 10 ns. Calculations of the excited-state potential energy surface found that both hydrogen and proton transfer are nearly isoenergetic separated by a 0.1 eV barrier. To further elucidate these dynamics, we now report a study with deuterated indigo, using resonance-enhanced multi-photon ionization and pump-probe spectroscopy with mass spectrometric isotopomer selection. From new calculations of the excited-state potential energy surface, we find sequential double-proton or hydrogen transfer, whereby the trajectory to the second transfer passes a second barrier and then encounters a conical intersection that leads back to the ground state. We find that deuteration only increases the excited-state lifetimes of the fast decay channel, suggesting tunneling through the first barrier, while the slower channel is not affected and may involve a different intermediate state.Graphical abstract

When Phenyl and Cyclopropyl Rings Meet: Spectroscopic Shifts and Conformational Questions.

The electronic and vibrational spectra of cyclopropylbenzene (CPB) and 1,3-bromocyclopropylbenzene (BrCPB) in the gas phase were investigated using quantum chemical calculations in combination with resonance-enhanced multi-photon ionization REMPI techniques including 1c-R2PI, UV-UV holeburning, and IR-UV ion depletion in the CH stretch region. The electronic spectra revealed the presence of a single conformer for both species, with the absence of any perpendicular conformer attributed to low computed barriers to conformer interconversion. Assignment of CPB to the bisected conformer was made through interpreting distinctive CH stretch bands in the IR-UV spectrum in conjunction with quantum chemical calculations. A local anharmonic model based on DFT calculations was adapted to reproduce the cyclopropyl CH stretch spectrum successfully. It was not feasible to definitively assign which bisected conformer of BrCPB was observed using vibrational information alone due to the close similarity of their predicted IR spectra. However, conformational sensitivity of the S1 ← S0 transition dipole moment (TDM) alignments leads to simulated rotational contours that display stark differences, which prompted assignment to the "B1" bisecting conformer with the cyclopropyl ring directed away from the bromine atom. The absence of the energetically comparable "B2" conformer is unexpected. The analysis of the convolution of aromatic and aliphatic modes serves as a basis for assignment in constrained aliphatic systems.

Visualizing vibrationally resolved attosecond time delay in resonance-enhanced multiphoton ionization of NO molecules

Visualizing vibrationally resolved attosecond time delay in resonance-enhanced multiphoton ionization of NO molecules

An efficient preparation of HD+ molecular ions in an ion trap by REMPI

We report a method of efficiently generating ultracold hydrogen molecular ions HD+ in a segmented linear ion trap. HD+ ions are created by [2+1′] resonance-enhanced multiphoton ionization (REMPI) of an HD supersonic molecular beam, and sympathetically cooled by laser-cooled Be+ ions. The HD+ ions are generated species-selectively using 201 nm and 395 nm pulse lasers with energies of 0.08 mJ (5.1 × 106 W/cm2) and 1.5 mJ (9.5 × 107 W/cm2) respectively. The average loading rate of 4.3(2) HD+ ions per second in the ion trap is determined by comparing the experimental images with the simulated images of Be+-HD+ bi-component Coulomb crystals. This method paves a way to produce the state-selected HD+ ions, which can improve the signal strengths of the selected rovibrational transitions of HD+ ions.