36 research outputs found
Untersuchungen zur Photodissoziationsdynamik von atmosphärischrelevanten Alkylketonen und Methylnitrit mittels Photofragment-Geschwindigkeitskartographie.
The photodissociation dynamics of trans-methyl nitrite and atmospherically relevant alkyl ketones in the near UV were studied by velocity map imaging.
The photolysis of trans-methyl nitrite excited to the S1 state with 0 and 1 quanta of excitation in the N=O-stretching mode, has been investigated. The primarily NO fragments were probed by (1+1) REMPI. The results demonstrate two dissociation pathways: a vibrationally non-adiabatic predissociation, and an adiabatic channel via tunneling through a potential barrier on the S1 potential energy surface. The O-NO bond dissociation energy was determined to be D = 38.8 kcal/mol and an S1 excited state lifetime of t = 350 fs was found.
The photodissociation dynamics of acetone-h6 excited within the first absorption band (340 - 230 nm) was investigated by detecting the nascent CH3 fragments in their ground state by (2+1) REMPI. The dissociation of acetone to CH3 and CH3CO fragments is not direct and proceeds via three pathways corresponding to three spectral regions of the excitation
wavelengths. In region 1 (333.5 - 305.3 nm), acetone is excited to the S1 state followed by internal conversion to the S0 state with unimolecular decay on it. Acetone excited in region 2 (305.3 - 260 nm) dissociates over a small
barrier on the T1 surface (“tight” transition state). The height of the barrier was measured to be 13.4 kcal/mol. In region 3 ( < 260 nm), secondary dissociation of CH3CO to CH3 and CO was observed. The
C-C bond dissociation energy
was measured to be D = 83.2 kcal/mol. The same experiments were carried out on acetone-d6 by probing the CD3 fragments.
Isotopic substitution did neither demonstrate any noticable effect on the photodissociation dynamics nor on the
C-C bond dissociation energy. This was interpreted as that the deuteration decreases the zero-point energy of acetone in the
T1 state and in the transition state. The rotational energy of the molecule itself and its internal rotors might lower the effective T1 barrier height. The dissociation limit
of acetyl-d3 was affected by the deuteration. The dissociation limit of acetyl-d3 was estimated to be 19.5 kcal/mol, that in acetyl-h3 is known to be 17 kcal/mol. The
increase upon deuteration may partially reflect the longer lifetime of the acetyl-d3 radical toward unimolecular dissociation.
The photolysis of methyl ethyl ketone via (a) CH3C(O)C2H5 + hv = CH3 + C2H5CO and (b) CH3C(O)C2H5 + hv = C2H5 + CH3CO channels was studied by probing CH3 via (2+1) REMPI for reaction (a), and C2H5 by (1+1) REMPI for reaction (b). The ion distribution was found to be independent of excitation energy. The present results do not provide enough data
for a quantitative analysis. Further experimental and theoretical investigations would be of great interest.Die Photodissoziationsdynamik von trans-Methylnitrit und anderen, atmosphärisch relevanten Alkylketonen wurde mittels Photofragment-Geschwindigkeitskartographie untersucht.
Die Photolyse von trans-Methylnitrit wurde im S1-Zustand verfolgt, wobei die N=O Streckschwingung 0 oder 1 Quant angeregt war. Die erzeugten NO-Fragmente wurden
durch (1+1)-REMPI detektiert. Zwei Dissoziationskanäle wurden beobachtet. 1) Es erfolgt eine schwingungsspezifische, nicht-adiabatische Prädissoziation oder 2) ein adiabatisches
Tunneln durch eine Potentialbarriere im S1-Zustand. Die O-NO Bindungsdissoziationsenergie wurde zu D = 38.8 kcal/mol und die Lebensdauer des angeregten Zustand zu t = 350 fs bestimmt.
Die Photodissoziationsdynamik von Aceton-h6 wurde zwischen 340 und 230 nm unterzucht. Es wurden die bei der Photodissoziation entstandenen CH3 Fragmente im
Schwingungsgrundzustand durch ein (2+1)-REMPI nachgewiesen. Die Dissoziation von Aceton in CH3 und CH3CO erfolgte nicht direkt, sondern abhängig vom Anregebereich über drei verschiedene Kanäle. Im Bereich von 333.5 bis 305.3 nm kehrte das photoangeregte Aceton durch Interne Konversion in
den S0-Grundzustand zurück. Zwischen 305.3 und 260 nm erfolgte der Dissoziationschritt über einen Übergangszustand entlang einer kleinen Energiebarriere im T1-Zustand (“tight” transition state), deren Höhe auf 13.4 kcal/mol extrapoliert
wurde. Bei Anregewellenlängen CH3 + CO verlief. Die
C-C Bindungsdissoziationsenergie wurde zu D = 83.2 kcal/mol bestimmt. An Aceton-d6 wurden vergleichbare Experimente durchgeführt und das CD3 Fragment detektiert. Es konnte kein Isotopeneffekt beobachtet werden, so dass angenommen wurde, dass die Deuterierung sowohl die Nullpunktsenergie des T1-Zustand als auch die des Übergangszustands absenkt. Die Barrierenhöhe im T1-Zustand könnte durch die Gesamtrotationsenergie des Moleküls und seine internen Rotatoren gemindert sein. Durch die Deuterierung wurde eine Verschiebung der Dissoziationsgrenze
für Acetyl-d3 auf 19.5 kcal/mol abgeschätzt, während die des Acetyl-h3 bei 17 kcal/mol liegt. Diese Zunahme könnte der längeren Lebensdauer des Acetyl-d3 Radikals zugeschrieben werden.
In Methylethylketon liegen zwei Photolysekanäle vor: (a) CH3C(O)C2H5 + hv = CH3 + C2H5CO und (b) CH3C(O)C2H5 + hv = C2H5 + CH3CO. Ersterer wurde anhand der entstandenen CH3 Fragmente durch ein (2+1)-REMPI untersucht. Bei letzterem wurden C2H5 Fragmente durch ein (1+1)-REMPI detektiert. Es wurde keine Abhängigkeit von Anregewellenlängen gefunden.
Die vorliegenden Daten erlaubten jedoch keine quantitative Analyse, hierzu wären weitere Experimente und theoretische Arbeiten notwendig
Tracking the surface atomic motion in a coherent phonon oscillation
X-ray photoelectron diffraction is a powerful tool for determining the
structure of clean and adsorbate-covered surfaces. Extending the technique into
the ultrafast time domain will open the door to studies as diverse as the
direct determination of the electron-phonon coupling strength in solids and the
mapping of atomic motion in surface chemical reactions. Here we demonstrate
time-resolved photoelectron diffraction using ultrashort soft X-ray pulses from
the free electron laser FLASH. We collect Se 3d photoelectron diffraction
patterns over a wide angular range from optically excited BiSe with a
time resolution of 140 fs. Combining these with multiple scattering simulations
allows us to track the motion of near-surface atoms within the first 3 ps after
triggering a coherent vibration of the A optical phonons. Using a
fluence of 4.2 mJ/cm from a 1.55 eV pump laser, we find the resulting
coherent vibrational amplitude in the first two interlayer spacings to be on
the order of 1 pm
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CAMP@FLASH: an end-station for imaging, electron- and ion-spectroscopy, and pump–probe experiments at the FLASH free-electron laser
The non-monochromatic beamline BL1 at the FLASH free-electron laser facility at DESY was upgraded with new transport and focusing optics, and a new permanent end-station, CAMP, was installed. This multi-purpose instrument is optimized for electron- and ion-spectroscopy, imaging and pump–probe experiments at free-electron lasers. It can be equipped with various electron- and ion-spectrometers, along with large-area single-photon-counting pnCCD X-ray detectors, thus enabling a wide range of experiments from atomic, molecular, and cluster physics to material and energy science, chemistry and biology. Here, an overview of the layout, the beam transport and focusing capabilities, and the experimental possibilities of this new end-station are presented, as well as results from its commissioning
The monochromator beamline at FLASH: performance, capabilities and upgrade plans
The monochromator beamline at the FLASH facility at DESY is the worldwide
first XUV monochromator beamline operational on a free electron laser
(FEL)source. Being a single-user machine, FLASH demands a high flexibility of
the instrumentation to fulfil the needs of diverse experiments performed by a
multidisciplinary user community. Thus, the beamline has not only been used for
high-resolution spectroscopy that it was originally designed for, but also for
pump-probe experiments controlling the temporal-spectral properties at moderate
resolution, and as a filter for high harmonics of the FEL at very low
resolution. The present performance and capabilities of the beamline are
discussed with emphasis on particularities arising from the nature of the FEL
source, and current developments are presented aiming to enhance its
capabilities for accommodating a wide variety of experiments