Photon-induced Fluorescence Spectroscopy Research Articles

Circular Dichroism in Fluorescence Emission Following the C 1s→π* Excitation and Resonant Auger Decay of Carbon Monoxide.

Dichroism in angle-resolved spectra of circularly polarized fluorescence from freely-rotating CO molecules was studied experimentally and theoretically. For this purpose, carbon monoxide in the gas phase was exposed to circularly polarized soft X-ray synchrotron radiation. The photon energy was tuned across the C 1s→π* resonant excitation, which decayed via the participator Auger transition into the CO+ A 2Π state. The dichroic parameter β1 of the subsequent CO+ (A 2Π → X 2Σ+) visible fluorescence was measured by photon-induced fluorescence spectroscopy. Present experimental results are explained with the ab initio electronic structure and dynamics calculations performed by the single center method. Our results confirm the possibility to perform partial wave analysis of the emitted photoelectrons in closed-shell molecules.

Elimination and migration of hydrogen in the vacuum-ultraviolet photodissociation of pyridine molecules

Elimination of the excited hydrogen atoms H(n), n = 4–7, and hydrogen migration in formation of the excited NH(A3Π) free radicals in the photodissociation of pyridine, C5H5N, molecules have been studied over the 17.5–70 eV photon energy range. In the measurements the photon-induced fluorescence spectroscopy technique has been applied. Both fragments are produced through excitation of pyridine molecules into higher-lying superexcited Rydberg or doubly excited states. The mechanisms for fragmentation of pyridine into H(n) and NH(A3Π) are discussed. Ab initio quantum chemical calculations have been performed to elucidate the hydrogen migration mechanism in the NH formation, which is not a self-contained unit in the structure of pyridine.

Hydrogen migration in photodissociation of the pyridine molecules

SynopsisPhotodissociation of the pyridine molecules has been studied experimentally over the 16-70 eV photon energy range. Formation of the excited NH(A3Π) radicals has been observed among several other atomic and dia-tomic fragments, using the photon-induced fluorescence spectroscopy. Since the pyridine molecules do not con-tain the NH structural units, this is an indication of the bond rearrangement processes associated with migration of the hydrogen atoms prior to dissociation. The plausible mechanisms for the NH formation will be discussed.

Hydrogen migration in formation of NH(A³Π) radicals via superexcited states in photodissociation of isoxazole molecules.

Formation of the excited NH(A(3)Π) free radicals in the photodissociation of isoxazole (C3H3NO) molecules has been studied over the 14-22 eV energy range using photon-induced fluorescence spectroscopy. The NH(A(3)Π) is produced through excitation of the isoxazole molecules into higher-lying superexcited states. Observation of the NH radical, which is not a structural unit of the isoxazole molecule, corroborates the hydrogen atom (or proton) migration within the molecule prior to dissociation. The vertical excitation energies of the superexcited states were determined and the dissociation mechanisms of isoxazole are discussed. The density functional and ab initio quantum chemical calculations have been performed to study the mechanism of the NH formation.

Formation of CN (B2Σ+) radicals in the vacuum-ultraviolet photodissociation of pyridine and pyrimidine molecules

Formation of the excited CN(B2Σ+) free radicals in the photodissociation of pyridine (C5H5N) and pyrimidine (C4H4N2) molecules was investigated over the energy ranges 16–27 and 14.7–25 eV, respectively. Photon-induced fluorescence spectroscopy was applied to detect the vibrationally and rotationally excited CN radicals by recording the B2Σ+→X2Σ+ emission bands (violet system). The measured dissociation yield curves demonstrate that the CN(B2Σ+) formation occurs via excitation of pyridine and pyrimidine molecules into higher-lying superexcited states. This is followed by rearrangement and isomerization of the excited molecules before dissociation. The vertical excitation energies of the superexcited states were determined and the probable dissociation mechanisms of both molecules are discussed.

Superexcited states in the vacuum-ultraviolet photofragmentation of isoxazole molecules

The photofragmentation of isoxazole molecules producing excited atomic and molecular fragments has been investigated over the energy range 16–50 eV, using photon-induced fluorescence spectroscopy. The following fragments have been identified by their fluorescence: the excited hydrogen atoms H(n), n = 3–7 and the diatomic CH(A2Δ, B2Σ−), CN(B2Σ+) and C2(d3Πg) fragments. The diatomic fragments are vibrationally and highly rotationally excited. The fragmentation yield curves for the CH(A2Δ), CN(B2Σ+), C2(d3Πg) and the H(n), n = 3–7 deduced from their emission yields, were obtained in the photon energy ranges from their appearance energies up to 50 eV. It is found that the fragmentation occurs via excitation into higher lying superexcited states of isoxazole. The vertical excitation energies of these states were determined and their possible fragmentation processes are discussed.

Photofragmentation of tetrahydrofuran molecules in the vacuum-ultraviolet region via superexcited states studied by fluorescence spectroscopy

Photofragmentation of tetrahydrofuran molecules in the vacuum-ultraviolet region, producing excited atomic and molecular fragments, has been studied over the energy range 14--68 eV using photon-induced fluorescence spectroscopy. Excited hydrogen atoms H($n$), $n$ $=$ 3--11, have been detected by observation of the ${\mathrm{H}}_{\ensuremath{\alpha}}$ to ${\mathrm{H}}_{\ifmmode \imath \else \i \fi{}}$ lines of the Balmer series. The diatomic CH(${A}^{2}\ensuremath{\Delta}$), CH(${B}^{2}{\ensuremath{\Sigma}}^{\ensuremath{-}}$) and ${\mathrm{C}}_{2}({d}^{3}{\ensuremath{\Pi}}_{g})$ fragments, which are excited to low vibrational and high rotational levels are identified by their ${A}^{2}\ensuremath{\Delta}\ensuremath{\rightarrow}{X}^{2}{\ensuremath{\Pi}}_{r}$, ${B}^{2}{\ensuremath{\Sigma}}^{\ensuremath{-}}\ensuremath{\rightarrow}{X}^{2}{\ensuremath{\Pi}}_{r}$ and ${d}^{3}{\ensuremath{\Pi}}_{g}\ensuremath{\rightarrow}{a}^{3}{\ensuremath{\Pi}}_{u}$ emission bands, respectively. Dissociation efficiency curves for CH$({A}^{2}\ensuremath{\Delta})$ and H($n$), $n$ $=$ 3--7, have been obtained in the photon energy ranges from their appearance thresholds up to 68 eV. The appearance energies for CH$({A}^{2}\ensuremath{\Delta})$ and H($n$), $n$ $=$ 3--7, have been determined and are compared with estimated fragmentation energy limits in order to discuss the possible fragmentation processes. In the present studies, superexcited states of tetrahydrofuran are found, which dissociate into the above excited atomic and molecular fragments.

Photoionization of Xe near 5s threshold: II. 5s- main line and satellites

Photon-induced fluorescence spectroscopy was applied to measure absolute fluorescence emission cross sections from the main 5s- and first satellite levels of XeII upon photoionization from XeI in a broad exciting-photon energy range (23–28 eV) with 20 meV bandwidth of the exciting radiation and in a narrow energy range (23.25–23.50 eV) with a bandwidth of 1.8 meV. The average fluorescence resolution amounted to 0.07 nm. Fluorescence emission cross sections were computed with taking into account many-electron correlations and doubly excited states. Computed cross sections, especially in integrated form, agree well with the results of experiment. Many features observed in the high resolution experiment are identified to originate from nf-Rydberg series.

Observation and identification of odd XeI Rydberg resonances near 5s-threshold

Photoionization cross sections for the production of main 5s line and some 5p4 5d/6s satellites of XeII in the region of the 5s-photoionization threshold have been measured by photon-induced fluorescence spectroscopy (PIFS) with extremely high primary resolution. The most pronounced features in the experimental spectra were interpreted on the basis of theoretical calculations.

Interference effects during the CO(1s−1π*) resonant excitation studied by polarization analysis of the CO+(A–X) fluorescence

The CO+ (A → X) fluorescence following the 1s → π* resonant excitation of the CO molecule and subsequent Auger decay has been investigated. The energy dependencies of the angular distribution parameters for the A 2Π(ν′) photoelectrons and for the subsequent A 2Π(ν′) → X 2Σ+(ν′′) fluorescence in the vicinity of the C*O and CO* resonances are predicted theoretically. These energy dependencies are due to the electronic states interference and lifetime vibrational interference. In order to test our predictions experimentally, polarization analysis of the CO+(A–X) fluorescence has been performed by photon-induced fluorescence spectroscopy.

Predominant dissociation of the CO*(D2Π)n(d/s)σ Rydberg states into atomic excited Rydberg fragments with the same effective principal quantum number

Dissociation of the CO molecule excited in the photon energy range between 20 eV and 25 eV is investigated by photon-induced fluorescence spectroscopy (PIFS) detecting dispersed fluorescence from C I* 2s22p1n(d/s) fragments in the spectral range of 115–135 nm. First experimental indications were found for a predominant dissociation of the CO*(D2Π)n(d/s)σ Rydberg states into the Rydberg states of the neutral C I atom conserving the effective principal quantum number n* = n − δ of the Rydberg electron. Such a process has as yet been verified only for the dissociating molecular Rydberg series (c4Σ−u)n(d/s)σg3Σ−u in O*2 (H Liebel et al 2000 Phys. Lett. 267 267). The selective dissociation of the molecular Rydberg states into atomic ones is interpreted by theoretical potential energy curves of the CO+ ion and molecular orbitals of Rydberg electrons computed within the single-centre approach.

Vibrational analysis of the N+2(C → X) fluorescence in the vicinity of the 1s → π* excitation

Fluorescence resulting from the 1s → π* excitation of the N2 molecule has been investigated experimentally and theoretically in the spectral range 165–203 nm. The experiment was performed by photon-induced fluorescence spectroscopy (PIFS) utilizing monochromatized synchrotron radiation from BESSY II with a bandwidth of 70 meV at 400 eV and a fluorescence resolution of 0.1 nm. The high fluorescence resolution enabled us to resolve individual C2Σ+u(v′) → X2Σ+g(v″) vibrational bands of the N+2 ion within the Δv = −6 and −7 vibronic band sequences. Unexpected features were observed in the individual fluorescence bands and are explained by calculations accounting for predissociation and molecular rotation.

Interaction between doubly-excited 4p4nℓn′ℓ′ resonances in KrI

Photoionization cross sections for the 4p4(3P)5s4P5/2,3/2,1/2 satellites and 4s → εℓ, 4p → εℓ main lines of Kr II were measured using the photon-induced fluorescence spectroscopy in the exciting-photon energy range between 27.80 eV and 29.44 eV with extremely narrow bandwidth (1.7 meV at 28.55 eV) of the monochromatized synchrotron radiation. The observed resonances were assigned to the 4p45s(4P1/2)np and 4p45s(2P3/2)np Rydberg series on the basis of the calculations including core relaxation and interaction between many resonances and many continua. The calculation shows that the resonance structure in the photoionization channels exists due to the 4p4(1D)5s2D5/26p3/2 and 4p4(1D)5s2D3/2 6p3/2,1/2 promoter states which strongly perturb the above Rydberg series.

Fluorescence emission following core excitations in the water molecule

Photon-induced fluorescence spectroscopy has been used to study the fragmentation of the water molecule at the O 1s edge. Fluorescence emission has been observed from the neutral fragments H, O and OH as well as from the ionic fragments O+ and OH+. The extracted fluorescence yields of the H Lyman-α emission and O 2p3(4S)3p(3P) → 2p3(4S)3s(3S) transitions show the same structures as the total ion yield spectrum but with different relative intensities. The most intense fluorescence emission is restricted to the region of the core excitations, while above the O 1s ionization limit the signal is much weaker (in the case of H) or below the detection limit (O, OH and OH+). The fluorescence emission is concluded to follow from the following general cascade: the core-excited states decay by resonant Auger transitions, the final states reached undergo dissociation into ionic and neutral fragments, and fluorescence occurs from excited fragments. In the case of the OH (A2Σ+ → X2Π) emission, the decay of core-excited states through soft x-ray emission may also be responsible for the observed fluorescence.

VUV photon induced fluorescence study of SF5CF3

The interaction of SF(5)CF(3) with vacuum-UV radiation has been investigated by photon induced fluorescence spectroscopy. Total fluorescence yield and dispersed fluorescence spectra of SF(5)CF(3) were recorded in the 200-1000 nm fluorescence window. In all cases, the fluorescence spectra resemble those of CF(3)X (X = H, F, Cl, and Br) molecules. At photon energies below 20 eV, the emission is attributed to the excited CF(3) and CF(2) fragments. The threshold for the CF(3) emission is 10.2 +/- 0.2 eV, giving an upper limit estimate for the SF(5)-CF(3) bond dissociation energy of 3.9 +/- 0.3 eV. The excitation functions of the CF(3) and CF(2) emissions were measured in the photon energy range 13.6-27.0 eV. The resonant structures observed in SF(5)CF(3) are attributed to electronic transitions from valence to Rydberg orbitals, following similar assignments in CF(3)X molecules. The photoabsorption spectrum of SF(5)CF(3) shows features at the same energies, indicating a strong contribution from Rydberg excitations.

Branching ratios in the radiative decay of helium doubly excited states

The doubly excited singlet states of He below the $N=2$ threshold may decay by autoionization or fluorescence. In the fluorescence decay channel, most decay cascades consist of emission of three photons, of which the first is a VUV photon, the second is in or near the visible range, and the last is another VUV photon. We have studied the fluorescence channel decay dynamics of the $(2,{0}_{\mathrm{n}})$, $(2,{1}_{\mathrm{n}})$ and $(2,\ensuremath{-}{1}_{\mathrm{n}})$ $^{1}\mathrm{P}$, $n=3--7$, states by wavelength dispersed photon-induced fluorescence spectroscopy. We have detected the photons in the second step of the cascade and determined the branching ratios for the strongest lines in this step. From these data we are able to calculate the branching ratios of the first step in the cascade. The results are in good agreement with calculations of the main decay channels of the higher resonances, but about 20--30 % lower, and so we are able to describe quantitatively the whole fluorescence cascade of the above-mentioned doubly excited states.

Interaction between resonances through autoionization continua near the 4s-threshold in KrII

The interaction between resonances through autoionization continua and the interaction between autoionization continua were investigated theoretically and experimentally for the photoionization process of the 4p- and 4s-shells and for the population of 4p4 (3P)5s 4PJ, 4p4 (3P)5s 2PJ and 4p4(3P)4d 4DJ satellites of KrII. Cross sections for the satellite production and the angular distribution parameter of the fluorescence radiation were measured by photon-induced fluorescence spectroscopy after excitationx with linearly polarized monochromatized synchrotron radiation at exciting-photon energies between 28.45 eV and 29.95 eV with an exciting-photon energy resolution of 10 meV (FWHM). Measured cross sections are in good agreement with the computed ones. A refined assignment of resonances in the proximity of the 4s14p65p resonance was performed. It was concluded that there is a strong influence of the core rearrangement, of the interaction between resonances through the autoionization continua and of the interaction between autoionization continua, on the investigated processes on the basis of the observed good overall agreement between the computed and measured quantities.

Absolute cross sections and branching ratios for the radiative decay of doubly excited helium determined by photon-induced fluorescence spectroscopy

The decay of doubly excited helium states below the N = 2 threshold, cascading radiatively over three steps, were investigated using the photon-induced fluorescence spectroscopy (PIFS) at BESSY II. Absolute cross sections as the product of the resonance excitation cross section of the doubly excited states and their fluorescence rate to decay into the singly excited 1sms(1S) and 1smd(1D) states were measured. The experiments showed that the (sp,2n+)(1P) states decay predominantly into the 1sns(1S) states, whereas the (pd,2n)(1P) states prefer to decay into the 1snd(1D) states. For the (sp,2n−)(1P) states with n = 4, 5 and 6 we observed a broad and complex decay pattern. In addition the angular distribution of the fluorescence radiation was measured. The results are in good agreement with theoretical predictions. Furthermore, the weakening of the LS coupling scheme and the mixing between singlet and triplet states in helium was confirmed by observation of the 1s6d(3D0,1,2) → 1s2p(3P0,1,2) transition on a doubly excited singlet state.

A new system for photon induced fluorescence spectroscopy applied to the study of doubly excited states of helium

A new system for photon induced fluorescence spectroscopy (PIFS) at the gas phase photoemission beamline (Elettra, Trieste) is described. It consists of a novel non-dispersive detector for VUV photons and a spectrometer for UV/visible light with cooled CCD detector. To illustrate the performance of the apparatus, the fluorescence decay of the doubly excited states of helium below the N = 2 ionization threshold has been studied. Fluorescence emission from both singlet and triplet states has been observed, and in particular weak visible emission resulting from decay of 7 3D and 8 3D triplet states confirmed the recently published assignment of these states. This illustrates how PIFS can be used to provide a direct and unambiguous verification of the assignments of states observed in photoabsorption.

Strongly perturbed Rydberg series originating from KrII 4p 45s ionic states

Photoionization cross-sections for the 4p 4 ( 3 P ) 5s 4 P 5 / 2 , 3 / 2 , 1 / 2 satellites and 4s, 4p main levels of Kr II in the exciting-photon energy range between 28.48 and 28.70 eV with extremely narrow bandwidth (1.7 meV at 28.55 eV) of the monochromatized synchrotron radiation were measured utilizing the photon-induced fluorescence spectroscopy. The observed resonances were assigned to the 4p 4 5s( 4 P 1 / 2 ) n p and 4p 4 5s( 2 P 3 / 2 ) n p Rydberg series on the basis of calculations performed with taking into account core relaxation and interaction between many resonances and many continua. The calculation shows that the resonance structure in the photoionization channels exists due to 4p 4 ( 1 D ) 5s 2 D 5 / 2 6 p 3 / 2 promoter state which also strongly perturbs the above Rydberg series.