Sum-frequency generation spectroscopy offers insights for oil and other applications – but assembling the right equipment can be a slippery problem. Laser spectroscopy enables the minute determination of molecular structure for a variety of applications ranging from biomedicine to materials science and space exploration. The Infrared Laser Spectroscopy Group in the chemistry department at the University of Cambridge uses several laser-based methods to study molecules in a wide range of situations. The researchers work closely on nonlinear laser spectroscopy techniques such as sum-frequency generation (SFG), which they use to gain insight into adsorption at interfaces on a molecular scale – of surfactants and polymers, for example. Nano- and picosecond lasers are used for these investigations, which typically are carried out in cooperation with industry, such as the oil and gas sector. The main laser equipment for the SFG spectroscopy study at the University of Cambridge was supplied by Ekspla. Photos courtesy of ©Dr. Robertas Kananavicius, Ekspla. “In addition to our international projects, which involve overseas visits by our research students, much of our work on surfaces and interfaces is strongly supported by industry,” the group has written in a mission statement on its website. When the department, in conjunction with a leading oil company, took on a significant research project designed ultimately to improve the efficiency of lubrication and to extend the life of the oil, the team needed a whole new suite of instruments. The project centered on the investigation of small friction-modifying molecules adsorbed onto metal surfaces; researching the adsorption of monolayer and submonolayer films in a dynamic wear environment is possible only with SFG spectroscopy. The research results could be applied to engines from those found in high-performance Formula 1 cars to those installed in standard road vehicles – particularly those with diesel engines, where oil performance presents a particular challenge. To test and analyze oil samples with precision, the department needed a so-phisticated SFG system to examine the way liquids and surfaces interact. These systems are expensive, so the department had to select one that would work not only for this research project, but also for the long term. Choosing a system Ekspla of Vilnius, Lithuania, makes solid-state picosecond and nanosecond laser systems, a broad selection of tunable laser sources and other tools, notably spectroscopy systems, for science and industry; its sole distributor in the UK is Acal BFi. When Cambridge faculty members turned to Ekspla and Acal BFi, they were looking for partners who could provide the right system as well as technical support in the form of advice, installation and service. Acal BFi still has ongoing business with the department, and Ekspla has been supportive in providing replacement or additional systems as necessary. The SFG spectroscopy system designed for the Cambridge research team comprised the following: (top left) Ekspla’s PL2231, a picosecond diode-pumped solid-state (DPSS) laser with upgrade to PG401-DFG2; (top right) the QE65 Pro scientific-grade spectrometer from Ocean Optics, with a fiber-coupled probe for Raman spectroscopy, along with Spectrasuite spectroscopy software; (bottom left) a 785-nm IR DPSS 50-mW Raman laser from CNI; and (bottom right) from Ekspla, a single-channel SFG spectrometer consisting of a mode-locked Nd:YAG laser PL2251B-20, parametric generator PG401-DFG2, harmonic box H400, single-channel SFG spectrometer and six-axis sample holder. Images courtesy of the companies. “The support throughout both the installation and running of the machine has been exceptionally good,” said Dr. Mike Casford, a postdoctoral researcher in the Cambridge chemistry department. “The expertise of the Ekspla service engineer is outstanding, whilst the support of Acal BFi has also been of an extremely high standard.” The company says that the relationship has been mutually beneficial: Working with the chemistry department has allowed Acal BFi to demonstrate its ability to understand complex needs and to work with its suppliers to ensure those needs are met. The company’s close relationship with Ekspla has resulted in a smooth process throughout, allowing the Cambridge team to focus on its research with complete confidence in the equipment. Equipment The main equipment – a picosecond diode-pumped solid-state (DPSS) laser and upgrade, and a single-channel SFG spectrometer consisting of a mode-locked Nd:YAG laser, a parametric generator with a tuning range of 0.42 to 16 µm, a harmonic box, a single-channel SFG spectrometer and a six-axis sample holder – was supplied by Ekspla. Acal BFi advised on and supplied a smaller system for sample testing, alongside fiber coupling and a small spectroscopy unit. Ocean Optics supplied a scientific-grade spectrometer and a fiber-coupled probe for Raman spectroscopy, along with SpectraSuite spectroscopy software; CNI supplied a 785-nm infrared DPSS 50-mW Raman laser. The smaller system allowed the researchers to obtain initial spectrometry data that helped them to decide whether to run the samples through the more sophisticated equipment. A comparison of spectral resolution from the Cambridge research: In femtosecond SFG (a), two peaks above 2900 cm−1 are unresolved; in Ekspla picosecond SFG (b), two peaks are clearly resolved at 2910 and 2930 cm−1. In a typical SFG setup, two laser beams – one fixed-frequency (532 nm) and one tunable across the IR (2.3 to 16 µm) – mix at a surface and generate an output beam with a frequency equal to the sum of the two input frequencies, producing a spectrum specific to the interface. The advantages of the technology for complex research projects of this sort include its sensitivity to monolayer surfaces and its ability to analyze sample surfaces in situ (for example, aqueous surfaces and in gases) with only minimal damage. These characteristics helped to fulfill the group’s requirements for a technically accurate, flexible and adaptable system. Laser systems with picosecond-pulse duration have inherently narrower line-width than femtosecond systems. The longer pulse gives increased spectral resolution in comparison with unmodified femtosecond-based systems and has helped the industrial partner to develop improved products. Results and publications The system’s accuracy and reliability allowed the preparation and publication of at least five papers within an 18-month time frame. The team has submitted two academic papers for publication1,2 and has delivered important information on the adsorption of monolayer and submonolayer films to its partner in the oil industry. “In both the long and short term, the purchase of this machine allows us to expand the frequency range that is accessible to us – and has dramatically increased the speed of data acquisition, resulting in a marked increase in the number of publications produced each year and also in the number of samples run for our sponsor,” Casford said. Meet the author Duncan Cooper is sales manager for photonics at Acal BFi in Milton Keynes, England; email: email@example.com. References 1. M.T.L. Casford and P.B. Davies. The structure of lipid bilayers adsorbed on activated carboxy-terminated substrates investigated by sum-frequency generation spectroscopy. Submitted to Journal of Physical Chemistry. 2. M.H. Wood et al (Nov. 12, 2013). Hexadecylamine adsorption at the iron oxide-oil interface. Langmuir, pp. 13735-13742.