Experimental reservoir computing with diffractively coupled VCSELs.

We present experiments on reservoir computing (RC) using a network of vertical-cavity surface-emitting lasers (VCSELs) that we diffractively couple via an external cavity. Our optical reservoir computer consists of 24 physical VCSEL nodes. We evaluate the system's memory and solve the 2-bit XOR task and the 3-bit header recognition (HR) task with bit error ratios (BERs) below 1% and the 2-bit digital-to-analog conversion (DAC) task with a root mean square error (RMSE) of 0.067.

Photoelectron spectrum of the pyridyl radical.

We report the photoelectron spectrum of the pyridyl radical (C5H4N), a species of interest in astrochemistry and combustion. The radicals were produced via hydrogen abstraction in a fluorine discharge and ionized with synchrotron radiation. Mass-selected slow photoelectron spectra of the products were obtained from photoelectron-photoion coincidence spectra. A Franck-Condon simulation based on computed geometries and vibrational frequencies identified contributions of the o- and p-pyridyl radicals. For the o-isomer an adiabatic ionisation energy of 7.70 eV was obtained, in excellent agreement with a computed value of 7.72 eV. The spectrum of o-pyridyl is characterized by a long progression in an in-plane bending mode and the N-C stretch that contains the radical site.

Impact of isoelectronic substitution on the excited state processes in polycyclic aromatic hydrocarbons: a joint experimental and theoretical study of 4a,8a-azaboranaphthalene.

Substituting CC with the isoelectronic BN units is a promising approach to modify the optoelectronic properties of polycyclic aromatic hydrocarbons. While computational studies have already addressed trends in the electronic structure of the various isosteres, experimental data are still scarce. Here, the excited state spectroscopy and dynamics of 4a,8a-azaboranaphthalene were studied by picosecond time-resolved photoionization in a supersonic jet and analyzed with the aid of XMS-CASPT2 and time-dependent DFT calculations. A resonance-enhanced multiphoton ionization spectrum (REMPI) reveals the S1 origin at  = 33 830 ± 12 cm-1. Several vibrational bands were resolved and assigned by comparison with the computations. A [1+1] photoelectron spectrum via the S1 origin yielded an adiabatic ionization energy of 8.27 eV. Selected vibrational bands were subsequently investigated by pump-probe photoionization. While the origin as well as several low-lying vibronic states exhibit lifetimes in the ns-range, a monoexponential decay is observed at higher excitation energies, ranging from 400 ps at +1710 cm-1 to 13 ps at +3360 cm-1. The deactivation is attributed to an internal conversion of the optically excited S1 state via a barrier that gives access to a conical intersection (CI) to the S0 state. The doping significantly changes the energetic ordering of CIs and lowers the corresponding energy barrier for the associated deactivation pathway, as revealed by nudged elastic band (NEB) calculations.

Injection locking and coupling the emitters of large VCSEL arrays via diffraction in an external cavity.

Networks of semiconductor lasers are the foundation of numerous applications and fundamental investigations in nonlinear dynamics, material processing, lighting, and information processing. However, making the usually narrowband semiconductor lasers within the network interact requires both high spectral homogeneity and a fitting coupling concept. Here, we report how we use diffractive optics in an external cavity to experimentally couple vertical-cavity surface-emitting lasers (VCSELs) in a 5×5 array. Out of the 25 lasers, we succeed to spectrally align 22, all of which we lock simultaneously to an external drive laser. Furthermore, we show the considerable coupling interactions between the lasers of the array. This way, we present the largest network of optically coupled semiconductor lasers reported so far and the first detailed characterization of such a diffractively coupled system. Due to the high homogeneity of the lasers, the strong interaction between them, and the scalability of the coupling approach, our VCSEL network is a promising platform for experimental investigations of complex systems, and it has direct applications as a photonic neural network.

Experimental demonstration of bandwidth enhancement in photonic time delay reservoir computing.

Time delay reservoir computing (TDRC) using semiconductor lasers (SLs) has proven to be a promising photonic analog approach for information processing. One appealing property is that SLs subject to delayed optical feedback and external optical injection, allow for tuning the response bandwidth by changing the level of optical injection. Here we use strong optical injection, thereby expanding the SL's modulation response up to tens of gigahertz. Performing a nonlinear time series prediction task, we demonstrate experimentally that for appropriate operating conditions, our TDRC system can operate with sampling times as small as 11.72 ps, without sacrificing computational performance.

IR/UV Double Resonance Study of the 2-Phenylallyl Radical and its Pyrolysis Products.

Isolated 2-phenylallyl radicals (2-PA), generated by pyrolysis from a nitrite precursor, have been investigated by IR/UV ion dip spectroscopy using free electron laser radiation. 2-PA is a resonance-stabilized radical that is considered to be involved in the formation of polycyclic aromatic hydrocarbons (PAH) in combustion, but also in interstellar space. The radical is identified based on its gas-phase IR spectrum. Furthermore, a number of bimolecular reaction products are identified, showing that the self-reaction as well as reactions with unimolecular decomposition products of 2-PA form several PAH efficiently. Possible mechanisms are discussed and the chemistry of 2-PA is compared with the one of the related 2-methylallyl and phenylpropargyl radicals.

Bonding in Low-Coordinated Organoarsenic and Organoantimony Compounds: A Threshold Photoelectron Spectroscopic Investigation.

Methyl and methylene compounds of arsenic and antimony have been studied by photoelectron photoion coincidence spectroscopy to investigate their relative stability. While for As both HAs=CH2 , As-CH3 and the methylene compound As=CH2 are identified in the spectrum, the only Sb compound observed is Sb-CH3 . Thus, there is a step in the main group 15 between As and Sb, regarding the relative stability of the methyl compounds. Ionisation energies, vibrational frequencies and spin-orbit splittings were determined for the methyl compound from photoion mass-selected photoelectron spectra. Although the spectroscopic results for organoantimony resemble those for the previously investigated bismuth compounds, EPR spectroscopic experiments indicate a far lower tendency for methyl transfer for Sb(CH3 )3 compared to Bi(CH3 )3 . This study concludes investigations on low-valent organopnictogen compounds.

Synthesis of Partially Fluorinated Alkyl Triflates by Electrochemical Fluorination (Simons Process).

A scalable straightforward synthesis of monofluoro- and difluoromethyl triflate CF3 SO2 OCH2 F (MH2F ) and CF3 SO2 OCHF2 (MHF2 ) through electrochemical fluorination (ECF, Simons process) of methyl triflate MH3 in anhydrous hydrogen fluoride at nickel anodes is presented. The ECF method is also feasible for the preparation of the deuterated analogues CF3 SO2 OCD2 F (MD2F ) and CF3 SO2 OCDF2 (MD2F ). Surprisingly, no H/D exchange occurs during ECF of CF3 SO2 OCD3 (MD3 ); this provides further evidence for a NiF3 /NiF4 -mediated ECF mechanism. The ECF of selected partially fluorinated ethyl triflates is described, and electrochemical fluorination of CF3 SO2 OCH2 CF3 (EH2F3 ) leads to the until now unknown chiral CF3 SO2 OCHFCF3 (EHFF3 ). The analogous fluoromethyl and fluoroethyl nonaflates are also accessible by ECF. This study contains detailed spectroscopic, structural, and thermal data on (fluoro)methyl and fluoro(ethyl) triflates.

Photoelectron Photoion Coincidence Spectroscopy of Biradicals.

The electronic structure of biradicals is characterized by the presence of two unpaired electrons in degenerate or near-degenerate molecular orbitals. In particular, some of the most relevant species are highly reactive, difficult to generate cleanly and can only be studied in the gas phase or in matrices. Unveiling their electronic structure is, however, of paramount interest to understand their chemistry. Photoelectron photoion coincidence (PEPICO) spectroscopy is an excellent approach to explore the electronic states of biradicals, because it enables a direct correlation between the detected ions and electrons. This permits to extract unique vibrationally resolved photoion mass-selected threshold photoelectron spectra (ms-TPES) to obtain insight in the electronic structure of both the neutral and the cation. In this review we highlight most recent advances on the spectroscopy of biradicals and biradicaloids, utilizing PEPICO spectroscopy and vacuum ultraviolet (VUV) synchrotron radiation.

Threshold photoelectron spectroscopy of trimethylborane and its pyrolysis products.

Trimethylborane (TMB) and its chemistry upon pyrolysis have been investigated by threshold photoelectron spectroscopy. TMB shows an unstructured spectrum and its adiabatic ionization energy (IEad) has been determined to be 9.93 ± 0.1 eV. Dissociative photoionization induces a methyl radical loss in TMB and the barrier to dissociation in the cation is measured to be 0.65 ± 0.1 eV. Upon pyrolysis methane loss dominates, leading to C2H5B, which can exist in five different isomeric structures. Quantum chemical calculations were used to investigate possible methane loss mechanisms as well as the isomerization pathways on the C2H5B potential energy surface. Through isomer-selective photoion mass-selected threshold photoelectron spectroscopy (ms-TPES) the two isomers CH3BCH2 and CH3CHBH were identified by their ms-TPE spectra and IEad values of 8.55 ± 0.02 eV and 8.73 ± 0.02 eV were determined, respectively. A second channel leading to the loss of ethene from TMB forms CH2BH, which exhibits an IEad value of 9.37 ± 0.03 eV. The reaction mechanism in the literature needs to be expanded by an additional methane loss from the intermediately formed ethyl methyl borane.

Time-resolved photoelectron spectroscopy of 4-(dimethylamino)benzethyne - an experimental and computational study.

We investigated the excited-state dynamics of 4-(dimethylamino)benzethyne (4-DMABE) in a combined theoretical and experimental study using surface-hopping simulations and time-resolved ionisation experiments. The simulations predict a decay of the initially excited S2 state into the S1 state in only a few femtoseconds, inducing a subsequent partial twist of the dimethylamino group within ∼100 fs. This leads to drastically reduced Franck-Condon factors for the ionisation transition to the cationic ground state, thus inhibiting the effective ionisation of the molecule, which leads to a vanishing photoelectron signal on a similar timescale as observed in our time-resolved photoelectron spectra. From the phototoelectron spectra, an adiabatic ionisation energy of 7.17 ± 0.02 eV was determined. The experimental decays match the theoretical predictions very well and the combination of both reveals the electronic characteristics of the molecule, namely the role of intramolecular charge transfer (ICT) states in the deactivation pathway of electronically excited 4-DMABE.

X-ray induced fragmentation of fulminic acid, HCNO.

The fragmentation of fulminic acid, HCNO, after excitation and ionization of core electrons was investigated using Auger-electron-photoion coincidence spectroscopy. A considerable degree of site-selectivity is observed. Ionization of the carbon and oxygen 1s electron leads to around 70% CH+ + NO+, while ionization at the central N-atom produces only 37% CH+ + NO+, but preferentially forms O+ + HCN+ and O+ + CN+. The mass-selected Auger-electron spectra show that these fragments are associated with higher binding energy final states. Furthermore, ionization of the C 1s electron leads to a higher propensity for C-H bond fission compared to O 1s ionization. Following resonant Auger-Meitner decay after 1s → 3π excitation, 12 different ionic products are formed. At the C 1s edge, the parent ion HCNO+ is significantly more stable compared to the other two edges, which we also attribute to the higher contribution of final states with low binding energies in the C 1s resonant Auger electron spectra.

Photoelectron spectroscopy and dissociative photoionization of fulminic acid, HCNO.

We report a joint experimental and computational study of the photoelectron spectroscopy and the dissociative photoionization of fulminic acid, HCNO. The molecule is of interest to astrochemistry and astrobiology as a potential precursor of prebiotic molecules. Synchrotron radiation was used as the photon source. Dispersive photoelectron spectra were recorded from 10 to 22 eV, covering four band systems in the HCNO cation, and an ionization energy of 10.83 eV was determined. Transitions into the Renner-Teller distorted X+2Π state of the cation were simulated using wavepacket dynamics based on a vibronic coupling Hamiltonian. Very good agreement between experiment and theory is obtained. While the first excited state of the cation shows only a broad and unstructured spectrum, the next two higher states exhibit a well-resolved vibrational progression. Transitions into the excited electronic states of HCNO+ were not simulated due to the large number of electronic states that contribute to these transitions. Nevertheless, a qualitative assignment is given, based on the character of the orbitals involved in the transitions. The dissociative photoionization was investigated by photoelectron-photoion coincidence spectroscopy. The breakdown diagram shows evidence for isomerization from HCNO+ to HNCO+ on the cationic potential energy surface. Zero Kelvin appearance energies for the daughter ions HCO+ and NCO+ have been derived.

Exploiting oscillatory dynamics of delay systems for reservoir computing.

Nonlinear dynamical systems exhibiting inherent memory can process temporal information by exploiting their responses to input drives. Reservoir computing is a prominent approach to leverage this ability for time-series forecasting. The computational capabilities of analog computing systems often depend on both the dynamical regime of the system and the input drive. Most studies have focused on systems exhibiting a stable fixed-point solution in the absence of input. Here, we go beyond that limitation, investigating the computational capabilities of a paradigmatic delay system in three different dynamical regimes. The system we chose has an Ikeda-type nonlinearity and exhibits fixed point, bistable, and limit-cycle dynamics in the absence of input. When driving the system, new input-driven dynamics emerge from the autonomous ones featuring characteristic properties. Here, we show that it is feasible to attain consistent responses across all three regimes, which is an essential prerequisite for the successful execution of the tasks. Furthermore, we demonstrate that we can exploit all three regimes in two time-series forecasting tasks, showcasing the versatility of this paradigmatic delay system in an analog computing context. In all tasks, the lowest prediction errors were obtained in the regime that exhibits limit-cycle dynamics in the undriven reservoir. To gain further insights, we analyzed the diverse time-distributed node responses generated in the three regimes of the undriven system. An increase in the effective dimensionality of the reservoir response is shown to affect the prediction error, as also fine-tuning of the distribution of nonlinear responses. Finally, we demonstrate that a trade-off between prediction accuracy and computational speed is possible in our continuous delay systems. Our results not only provide valuable insights into the computational capabilities of complex dynamical systems but also open a new perspective on enhancing the potential of analog computing systems implemented on various hardware platforms.

Ammonia Borane, NH3 BH3 : A Threshold Photoelectron-Photoion Coincidence Study of a Potential Hydrogen-Storage Material.

We have investigated the photoionization of ammonia borane (AB) and determined adiabatic ionization energy to be 9.26±0.03 eV for the X+ 2 E←X 1 A1 transition. Although the threshold photoelectron spectrum appears at first glance to be similar to the one of the isosteric ethane, the electronic situation differs markedly, due to different orbital energies. In addition, an appearance energy AE0K (NH3 BH3 , NH3 BH2 + )= 10.00±0.03 eV has been determined, corresponding to the loss of a hydrogen atom at the BH3 -site. From the data, a 0 K bond dissociation energy for the B-H bond in the cation of 71.5±3 kJ mol-1 was derived, whereas the one in the neutral compound has been estimated to be 419±10 kJ mol-1 .

Transmission delays and frequency detuning can regulate information flow between brain regions.

Brain networks exhibit very variable and dynamical functional connectivity and flexible configurations of information exchange despite their overall fixed structure. Brain oscillations are hypothesized to underlie time-dependent functional connectivity by periodically changing the excitability of neural populations. In this paper, we investigate the role of the connection delay and the detuning between the natural frequencies of neural populations in the transmission of signals. Based on numerical simulations and analytical arguments, we show that the amount of information transfer between two oscillating neural populations could be determined by their connection delay and the mismatch in their oscillation frequencies. Our results highlight the role of the collective phase response curve of the oscillating neural populations for the efficacy of signal transmission and the quality of the information transfer in brain networks.

Photoelectron spectroscopy in molecular physical chemistry.

Photoelectron spectroscopy has long been a powerful method in the toolbox of experimental physical chemistry and molecular physics. Recent improvements in coincidence methods, charged-particle imaging, and electron energy resolution have greatly expanded the variety of environments in which photoelectron spectroscopy can be applied, as well as the range of questions that can now be addressed. In this Perspectives Article, we focus on selected recent studies that highlight these advances and research areas. The topics include reactive intermediates and new thermochemical data, high-resolution comparisons of experiment and theory using methods based on pulsed-field ionisation (PFI), and the application of photoelectron spectroscopy as an analytical tool to monitor chemical reactions in complex environments, like model flames, catalytic or high-temperature reactors.

Photoelectron spectroscopy of low valent organophosphorus compounds, P-CH3, H-PCH2 and PCH2.

We report the mass-selected slow photoelectron spectra of three reactive organophosphorus species, PCH2, and the two isomers, methylenephosphine or phosphaethylene, HPCH2 and methylphosphinidine, P-CH3. All spectra were recorded by double imaging photoelectron-photoion coincidence spectroscopy (i2PEPICO) using synchrotron radiation and all species were generated in a flow reactor by the reaction of trimethyl phosphine with fluorine atoms. Adiabatic ionisation energies of 8.80 ± 0.02 eV (PCH2), 10.07 ± 0.03 eV (H-PCH2) and 8.91 ± 0.04 eV (P-CH3) were determined and the vibronic structure was simulated by calculating Franck-Condon factors from optimised structures based on quantum chemical methods. Observation of biradicalic P-CH3 isomer with its triplet ground state is surprising because it is less stable than H-PCH2.

The gas-phase infrared spectra of the 2-methylallyl radical and its high-temperature reaction products.

The resonance-stabilized 2-methylallyl radical, 2-MA, is considered as a possible intermediate in the formation of polycyclic aromatic hydrocarbons (PAHs) in combustion processes. In this work, we report on its contribution to molecular growth in a high-temperature microreactor and provide mass-selective IR/UV ion dip spectra of the radical, as well as the various jet-cooled reaction products, employing free electron laser radiation in the mid-infrared region. Small (aromatic) hydrocarbons such as fulvene, benzene, styrene, or para-xylene, as well as polycyclic molecules, like (methylated) naphthalene, were identified with the aid of ab initio DFT computations. Several reaction products differ by one or more methyl groups, suggesting that molecular growth is dominated by (de)methylation in the reactor.

Photodissociation of the trichloromethyl radical: photofragment imaging and femtosecond photoelectron spectroscopy.

Halogen-containing radicals play a key role in catalytic reactions leading to stratospheric ozone destruction, thus their photochemistry is of considerable interest. Here we investigate the photodissociation dynamics of the trichloromethyl radical, CCl3 after excitation in the ultraviolet. While the primary processes directly after light absorption are followed by femtosecond-time resolved photoionisation and photoelectron spectroscopy, the reaction products are monitored by photofragment imaging using nanosecond-lasers. The dominant reaction is loss of a Cl atom, associated with a CCl2 fragment. However, the detection of Cl atoms is of limited value, because in the pyrolysis CCl2 is formed as a side product, which in turn dissociates to CCl + Cl. We therefore additionally monitored the molecular fragments CCl2 and CCl by photoionisation at 118.2 nm and disentangled the contributions from various processes. A comparison of the CCl images with control experiments on CCl2 suggest that the dissociation to CCl + Cl2 contributes to the photochemistry of CCl3.