Experimental Raman (5) Flashcards
major advantages IR
Fundamental vibrations, molecular ID, libraries, instrumental simplicity
major advantages Raman
narrower line widths, fibers and remote sampling, low frequency modes, H2O compatible, non-invasive sampling, enhancement mechanisms
Disadvantages IR
broader line widths, sample difficulty, water interferences
disadvantages Raman
low sensitivity, fluorescent interferences
Main components that make up a generic Raman spectrometer
detection, collection, excitation
Main characteristics of Raman lasers
wide range of excitation wavelengths possible (UV to Near IR)
Why are CW lasers preferable to pulsed lasers?
pulsed laser peak power is too high, sample damage, photochemistry concerns
Why is it necessary to filter the laser lines?
filter out extra atomic laser plasma lines for ion lasers
Physical principles of ion lasers
DC electron discharge through low pressure Ar or Kr forms a plasma magnetically confined within resonant cavity; one end of cavity contains totally reflecting (1005) retro-reflector; other end of cavity contains partially reflecting (95%) output coupler; only a few transitions last with sufficient gain to superpower CW laser output
What are common Ar wavelengths used as raman excitation lines?
514.5, 488, 351.1
Common Kr wavelengths used as Raman excitation lines
676.4, 647.1, 413.1
What are the main advantages of ion lasers?
What are the main disadvantages of ion lasers?
most power used to create ions, not excite laser transition; only 0.01-0.05% incident power is converted to laser output; low efficiency means high electrical power and cooling requirements;
power needed for small frame ion laser
208 V 3-phase, 10-15 kW total
power needed large frame ion laser
408 V 3-phase, 50-60 kW total
What are the physical principles of He-Ne lasers?
tasing transition is an Ne atom so power not consumed to create excited ions, energy transferred to Ne atoms before lasing, He electrons and ions carry current
What is the He-Ne wavelength used as a raman excitation line?
632.8
Advantages He-Ne lasers
lasing process much more efficient than ion lasers; ordinary 110 V power and air cooling sufficient; smaller and less expensive than ion lasers; laser wavelength well match to Si-based CCD Detectors
Disadvantages He-Ne lasers
only single lasing wavelength supported; output optical power much lower for He-Ne lasers than for ion lasers (< 50 mW); must filter additional atomic emission lines
What are the physical principles of near IR diode lasers?
doped semiconductors form a junction between p type (holes) and n type (electron) materials; electrons injected into conduction band of p type material, combine with holes, release energy equal to or exceeding band gap of material; recombination annihilates electron/hole pair, energy releases as light; pn junction functions as a light emitting diode, converts current to light
What are the most common near IR diode wavelengths used as Raman excitation lines?
670 to 865
What are the main advantages of near IR diode lasers?
high efficiency minimizes power and cooling requirements; very small footprint; wide wavelength range available in mid to near IR; inexpensive; useful power range of .1 to 1 watt
disadvantages of diode lasers
broad gain curve results in wavelength drift; mode structure difficult to stabilize resulting in uncertainty in output wavelength and observed shift; high divergence = less easily filtered and focused
