Available Equipment (ELI-ERIC)

The Gas Phase Reaction Control (GPRC) setup is suitable for pump -probe experiments in gas phase. The focused ultrashort laser pulses generate plasma in which radicals are generated from small molecules in the reaction chamber. The emitted light can be monitored from the excited radicals in the first few hundred nanosecond time range.

Brief description of the available set up

The Gas Phase Reaction Control (GPRC) setup is suitable for pump -probe experiments in gas phase. The focused ultrashort laser pulses generate plasma in which radicals are generated from small molecules in the reaction chamber. The emitted light can be monitored from the excited radicals. Following the ultrashort excitation pulse, the spectral changes in the emitted light can be monitored in the first few hundred nanosecond time range.

Description of key areas of science

The GPRC setup is suitable for different pump and probe experiments with a femtosecond NIR or attosecond XUV pump and emission probe. The combination of the femtosecond NIR or attosecond XUV pulses as pump could be combined with picosecond THz pump pulses. This could provide certain chemical control in the reactions changing the orientation of the small organic molecules before excitation. The diatomic radicals for example CN, CH and OH can be studied in excited states. Their spectral characterisation is important to different fields of science including astrophysics, plasma physics, laser induced breakdown spectroscopy (LIBS), analytical chemistry, combustion research and environmental science. The high spectral and temporal resolution allow the characterisation of the plasma conditions and the changes of the rotational and vibrational states of these radicals. The chirp control of the ultrashort pulses or the THz pump orienting the molecules could alter the composition of the resulting components in gas phase that are important for chemical dynamics research. The setup is commissioned with the attenuated beam of the SYLOS laser for NIR pump-probe, but the mobile station can be moved to an IR-XUV or IR-XUV-THz output of an HHG beamline.

Full description of system:

The SYLOS beam (890 nm, ~1 mJ/pulse, 1 kHz) is focused with a spherical mirror in a vacuum chamber in which small organic molecules are introduced with a piezo valve from organic vapours or pre-mixed gases. The motorised mirror mount provides fine positioning for plasma generation.  The operation of the piezo valve can be controlled with the opening voltage (0-200 V) and opening time (2-400 μs).  The experimental vacuum chamber where the photodissociation/reaction takes place at low pressure is monitored with two spectrometers to obtain information about the photodissociation processes. The molecules are introduced into the vacuum chamber via adiabatic expansion (supersonic pulsed valve) to ensure that the molecules are cold and in their ground vibrational and rotational states at the beginning of the reaction. The visible light from the photofragments is imaged onto a high-resolution Fourier transform spectrometer (Bruker Vertex80) equipped with a PMT detector. The spectrometer is capable of resolving rotational and vibrational line spacing in the UV-visible range with high spectral and temporal resolution (0.05 cm-1 and 2.5 ns). The temporal resolution (2.5 ns) is provided with the  step scan capability of the spectrometer, which allows to obtain time-resolved spectra. The step scan capability of the setup allows to obtain the nascent vibrational and rotational energy distribution from the dissociation process without the impact of molecular collisions following the photodissociation. The overview spectra of the photofragments can be monitored with an Ocean Optics QEPro spectrometer in the 200-1000 nm spectral range.

Future experiments with attosecond XUV and picosecond THz pump pulses will be suitable for investigating the THz control in chemical reactions and photodissociation processes. The high pulse energy from the SYLOS laser will be used to generate XUV laser pulses at sufficiently high energy that is able to break chemical bonds from single or multiple absorption of photons. The high pulse energy (40 mJ) will ensure the generation of the sufficient number of XUV photons (at least 1010 photons/second) that will be able to photolyze large number of molecules for measuring the energy disposal of the neutral molecular fragments with high energy resolution. The high pulse energy will be also used to produce very strong terahertz pulses (narrow band up to 2-400 kV/cm) to provide an optical electric field bias during photodissociation. This optical bias will be a key experimental tool to control energy disposal in molecular fragments via a quasi-DC STARK effect (time scale of terahertz pulse is long compared to the time scale of photodissociation).

 Figure 1 GPRC experimental setup and detection system at SYLOS beamline

Available sample delivery systems and target systems

The sample delivery is via introducing the molecules with a piezo valve that is synchronised with the SYLOS laser. The piezo valve can be positioned in x-y-z directions.

The laser beam is focused in the centre of the vacuum chamber with a spherical mirror and the molecules can be introduced into the plasma via position control of the piezo valve. 

Available detection and observation systems

Bruker Vertex80 FTUV-MIR spectrometer with a PMT detector is used for the measurement of the emitted light. The spectrometer is capable of resolving rotational and vibrational line spacing in the UV-visible range with high spectral and temporal resolution (0.05 cm-1 and 2.5 ns). The temporal resolution (2.5 ns) is provided with the step scan capability of the spectrometer, which allows to obtain time-resolved spectra. The overview spectra of the photofragments can be monitored with an Ocean Optics spectrometer in the 200-1000 nm spectral range.

References

 [1] Formation of CN Radical from Nitrogen and Carbon Condensation and from Photodissociation in Femtosecond Laser-Induced Plasmas: Time-Resolved FT-UV–Vis Spectroscopic Study of the Violet Emission of CN Radical; K. Mogyorosi, K. Sarosi, I. Seres, P. Jojart, M. Fule, and V. Chikan; The Journal of Physical Chemistry A 2020 124 (14), 2755-2767 DOI: 10.1021/acs.jpca.0c00361; https://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.jpca.0c00361&ref=pdf

[2] Direct Production of CH(A²Δ) Radical from Intense Femtosecond Near-IR Laser Pulses;
K. Mogyorosi, K. Sarosi, and V. Chikan; The Journal of Physical Chemistry A 2020 124 (40), 8112-8119; DOI: 10.1021/acs.jpca.0c05206; https://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.jpca.0c05206&ref=pdf