Mass Resolved Multiphoton Ionization Research Articles

Formation of highly excited iodine atoms from multiphoton excitation of CH3I.

Mass resolved REMPI spectra, as well as CH3+and I+ ion and photoelectron images, were recorded for two-photon resonant excitations of CH3I via s, p and d Rydberg states (CH3I**) in the excitation region of 55 700 to 70 000 cm-1. Photoelectron (PE) and ion kinetic energy release spectra (KERs) were derived from the images. The data revealed that after the two-photon resonant excitation, an additional photon is absorbed to form one or more superexcited state(s) (CH3I#), followed by branching into three pathways. The major one is the dissociation of CH3I# to form excited Rydberg states of iodine atoms (I**) along with CH3(X), a phenomenon not commonly observed in methyl halides. The second (minor) pathway involves autoionization of CH3I# towards CH3I+(X), which absorbs another photon to form CH3+ along with I/I* and the third one (minor) is CH3I# dissociation towards the ion pair, CH3+ + I-, prior to I- electron ejection. Furthermore, one-photon non-resonant dissociation of CH3I to form CH3(X) and I/I* prior to three-photon ionization of the fragments is also detected.

Multiphoton Rydberg and valence dynamics of CH3Br probed by mass spectrometry and slice imaging.

The multiphoton dynamics of CH3Br were probed by Mass Resolved MultiPhoton Ionization (MR-MPI), Slice Imaging and Photoelectron Imaging in the two-photon excitation region of 66 000 to 80 000 cm-1. Slice images of the CH3+ and Br+ photoproducts of ten two-photon resonant transitions to np and nd Rydberg states of the parent molecule were recorded. CH3+ ions dominate the mass spectra. Kinetic energy release spectra (KERs) were derived from slice and photoelectron images and anisotropy parameters were extracted from the angular distributions of the ions to identify the processes and the dynamics involved. At all wavelengths we observe three-photon excitations, via the two-photon resonant transitions to molecular Rydberg states, forming metastable, superexcited (CH3Br#) states which dissociate to form CH3 Rydberg states (CH3**) along with Br/Br*. A correlation between the parent Rydberg states excited and CH3** formed is evident. For the three highest excitation energies used, the CH3Br# metastable states also generate high kinetic energy fragments of CH3(X) and Br/Br*. In addition for two out of these three wavelengths we also measure one-photon photolysis of CH3Br in the A band forming CH3(X) in various vibrational modes and bromine atoms in the ground (Br) and spin-orbit excited (Br*) states.

Vibrational and Electronic Spectroscopy of the 4-Hydroxystyrene−CO2 Cluster and Its Hydrate: A para-Coumaric Acid Impostor

We report on the results of high-resolution spec-troscopic studies on the 4-hydroxystyrene-CO(2) cluster. We show that these clusters are generated upon heating of para-coumaric acid, the chromophore of the photoactive yellow protein (PYP), as the result of a thermal decarboxylation process. Since the mass of the cluster and the starting material are the same, standard mass-resolved multiphoton ionization spectroscopic methods do not suffice to distinguish these clusters from para-coumaric acid. Instead, more advanced methods that include various UV and IR depletion methods need to be applied. These methods, in combination with quantum chemical calculations, enable us to unravel the structural and spectroscopic properties of 4-hydroxystyrene-CO(2) as well as of its hydrate, 4-hydroxystyrene-CO(2)-H(2)O.

Mass resolved MPI spectra of methyl iodide in the 430–490 nm region

The mass resolved multiphoton ionization (MPI) spectra of methyl iodide were obtained in the 430–490 nm region using a time-of-flight (TOF) mass spectrometer. They have the same vibrational structure, which testifies that the fragment species, in the wavelength region under study, are from the photodissociation of multiphoton ionized molecular parent ions. Some features in the spectra are identified as three-photon excitations to 6 p and 7 s Rydberg states of methyl iodide. Two new vibrational structures of some Rydberg states are observed. The mechanism of ionization and dissociation is also discussed.

On the electronic states and photochemistry of simple alkyl amines

This paper extends recent work on the excited states of methyl amine to some other simple amines. Jet-cooled mass-resolved multiphoton ionization spectroscopy is reported for methyl amine, dimethyl amine, trimethyl amine, ethyl amine, diethyl amine, and triethyl amine. A set of high-level calculations for methyl amine supports the assignment of S1 as a 3s Rydberg state by predicting the experimental transition energy to within about 1000 cm−1 after including vibrational zero-point energies for each potential energy surface. Perturbations observed in the experimental S1←S0 spectrum prompt a series of calculations which implicate a dissociative 3s Rydberg state in both the perturbation in the spectrum and in the photodissociation of methyl amine at these energies. A series of excited state calculations performed at the ground state geometry for methyl amine, dimethyl amine and trimethyl amine indicates the relative energies of the bound 3s and 3p Rydberg states as well as the first dissociative 3s Rydberg state. Taken together, this work presents a picture of the S1 and S2 excited states in which dissociative low-lying Rydberg states play a significant role in both the spectroscopy and photochemistry of the simple amines.

Resonance enhanced multiphoton ionization spectroscopy of carbonyl sulphide

Rydberg excited states of the OCS molecule in the energy range 70500–86000 cm−1 have been investigated via the two and three photon resonance enhancements they provide in the mass resolved multiphoton ionization (MPI) spectrum of a jet-cooled sample of the parent molecule. Spectral interpretation has been assisted by companion measurements of the kinetic energies of the photoelectrons that accompany the various MPI resonances. The present study supports the earlier conclusions of Weinkauf and Boesl [J. Chem. Phys. 98, 4459 (1993)] regarding five Rydberg origins in the 70500–73000 cm−1 energy range, attributable to, respectively, states of 3Π, 1Π, 3Δ, 1Δ and 1Σ+ symmetry arising from the 4pλ←3π orbital promotion. We also identify a further 21 Rydberg origins at higher energies. These partition into clumps with quantum defects ca. 3.5 and 4.5, which we associate with the orbital promotions npλ←3π (n=5,6), and others with near integer quantum defect which are interpretable in terms of excitation to s,d and (possibly) f Rydberg orbitals. We also identify MPI resonances attributable to CO(X 1Σ+) fragments and to S atoms in both their ground (3P) and excited (1D) electronic states. Analysis of the former resonances confirms that the CO(X) fragments resulting from one photon dissociation of OCS at excitation wavelengths ca. 230 nm are formed with a highly inverted, bimodal rotational state population distribution, whilst the latter are consistent with previous reports of the wavelength dependence for forming ground and excited state S atoms in the near uv photolysis of OCS.

Resonance enhanced multiphoton ionization spectroscopy of carbon disulphide

Rydberg excited states of the CS2 molecule in the energy range 56 000–81 000 cm−1 have been further investigated via the two and three photon resonance enhancements they provide in the mass resolved multiphoton ionization (MPI) spectrum of a jet-cooled sample of the parent molecule. Spectral interpretation has been aided by parallel measurements of the kinetic energies of the photoelectrons that accompany the various MPI resonances. Thus we have been able to extend, and clarify, previous analyses of the tangled spin–orbit split vibronic structure associated with the 3Πu and 1Πu states derived from the configuration [2Πg]4pσu and the 3Δu, 1Δu, and 1Σ+u states resulting from the configuration [2Πg]4pπu, and to deduce an approximate wave number for the origin of the hitherto unidentified 3Σ+u state derived from this same configuration. Moving to higher energies we are able to locate, unambiguously, the origins of the next (n=5) members of four of these [2Πg]np Rydberg series, and to identify extensive series based on the presumed Rydberg configurations [2Πg]nsσg and [2Πg]nfλu with, in both cases, n≤10. We also identify MPI resonances attributable to CS(a 3Π) fragments, to ground state C atoms, and to S atoms in both their ground (3P) and excited (1S) electronic states. Analysis of the former resonances indicates that the CS(a 3Π) fragments resulting from two photon dissociation of CS2 at excitation wavelengths around 300 nm are formed with substantial rovibrational excitation.

Observation of the PF2 radical by resonance enhanced multiphoton ionization spectroscopy

We have observed nine new excited electronic states of the PF2 radical in the wave number range 50 000–71 000 cm−1 via the two photon resonance enhancements they provide in mass resolved multiphoton ionization spectroscopy. We also report ab initio calculations which yield optimized energies, geometries, and vibrational frequencies for the ground states of PF2 and of the PF+2 ion. Energetic considerations, and the observation that the frequencies of the ν1′ (symmetric stretch) and ν2′ (bending) modes in these excited states closely parallel those of the ground state ion, lead us to conclude that all nine excited states are Rydberg states belonging to series that converge to the ground state ionization limit. Polarization analyses and band contour simulations allow us to identify the electronic symmetries of the first two of these newly identified states (2B1 and 2A1, respectively) and to provide an estimate of the excited state geometries [r(P–F)≊1.49 Å, ∠F–P–F≊109°].

Photoreactivity of Molecular Aggregates

AbstractThe spectroscopy and photoreactivity of atmospheric trace gases embedded in clusters is examined in this work. Perturbations induced by intermolecular interactions are obtained in order to evaluate the role that the heterogeneous photochemistry of such species could play in the chemical balance of the atmosphere. – Spectroscopic effects are monitored using aggregates produced in supersonic jets and measured by high resolution Fourier Transform direct ultraviolet absorption techniques. These spectra are compared with those of the isolated molecule to extract shifts and intensity changes when the chromophore is clustered. Photochemical studies involve laser pump‐probe experiments with mass resolved multiphoton ionization detection. Changes in the isolated molecule photochemistry are monitored as a function of cluster formation. – This paper discusses the methodology developed as well as the specific example of clusters of ozone with atmospheric trace gases.

ChemInform Abstract: Multiphoton Spectroscopy of X‐NO (X: Kr, Xe, CH4) van der Waals Molecules.

AbstractThe spectra of the title species are investigated at wavelengths near the C2π ← X2π1/2 transition of nitric oxide by mass‐resolved multiphoton ionization (MPI) spectroscopy.

Multiphoton spectroscopy of X–NO (X=Kr, Xe, CH4) van der Waals molecules

The vibronic spectroscopy of the van der Waals species KrNO, XeNO, and CH4NO has been investigated via mass-resolved multiphoton ionization (MPI) spectroscopy. Spectra are presented for the two-photon excited electronic transition of the various complexes associated with the C 2Π (v=0) state of nitric oxide. Dissociation pathways are analyzed for XeNO, which appears at the Xe+ mass following MPI. Trends in bonding and spectroscopic constants are discussed.