This thesis is divided into two sections. The first section focuses on the IR spectroscopic determination of the CD/CH vibrational energy spacings of the quasilinear HCCN and DCCN nu5 bending mode. We have developed an in-house program to analyze and assign the HCCN and DCCN spectra. The program has three functions, namely, spectral contour simulation, P and R transition strength calculation, and diagnostic least squares fitting. The spectral contour simulation was the most valuable tool. It generated the Q contours of transitions between various states for comparison with the observed spectra. Overall, we have successfully assigned the HCCN and DCCN nu1+2nu 5+/-2←nu5+/-1, nu 1+3nu5+/-3←2nu5 +/-2 combination bands and the HCCN nu1+3nu 5+/-3←3nu5+/-3 hot band. Using this assignment with the previous published hot band analysis, 18,19 we determined the HCCN 2nu5+/-2←nu 5+/-1, 3nu5+/-3←2nu 5+/-2 and DCCN 2nu5+/-2 ←nu5+/-1 vibrational energy gaps to be 212.821, 272.864 and 133.106 cm-1, respectively. The second part of the thesis focuses on the design and development of a supersonic jet cooling system. This system aims to expand our high-resolution IR spectroscopic work to cover much larger and more non-rigid species. A similar attempt was made a decade ago; however, the prototype was not sufficiently reliable to pass the testing stage. This project expands on our previous effort and focuses on designing a more reliable and better-performing system. We replaced the home-made pulsed valve with a more reliable commercial valve and greatly reduced the maintenance time and enhanced the ease of operation. To increase the efficiency of radical generation, we adopted the novel slit discharge method25 in place of the photolysis approach. Finally, we used a more compact and stable Herriott multipass cell design to overcome the signal broadening and averaging effects introduced by the previous White-type multipass cell. Overall, we have made much progress in enhancing the stability and performance of the system.