laser - induced fluorescence ( lif ) is a technique that relies on the measurement of the excited state fluorescence that results from photoexcitation .
the excitation source for molecular lif is typically a tunable dye laser so that selective excitation and high resolution can be achieved .
combined with the supersonic - jet expansion technique , lif has been used to study the excited state structures of molecules and the ultra - fast dynamics of molecular systems .
lif has also been successfully applied for quantitative measurement of concentrations in combustion , plasma , spray , and flow phenomena , in some cases visualizing concentrations down to nanomolar levels .   unlike other analytical spectroscopy techniques , where ready - to - use commercial spectrometers have everything for the sample detection in a package , laboratories usually design and set up the lif experimental instruments by themselves in order to satisfy individual research needs .
in addition to hardware installation , lif also requires synchronized and automated control software for laser function , data acquisition , and communication between instruments [ 3 , 4 ] .
most laboratories develop homemade programs for their experiments using different programming languages , such as visual c++ , labview , visual basic , or turbo pascal , but text - based languages have the shortcomings of long developing cycles and difficulties in maintenance and expansion .
, usa ) is a graphical programming tool which can be employed to develop sophisticated measurements [ 68 ] and control [ 9 , 10 ] systems using intuitive graphical icons and wires that resemble a flowchart instead of written lines of text [ 1116 ] . in this paper ,
this system can simultaneously control the scanning of a dye laser and a monochromator , the operation of a molecular pump , and data acquisition .
an application of this program in the lif spectroscopy examination of -methylnaphthalene is shown in this paper .
there are two different kinds of spectra in lif spectroscopy , dispersed spectra , and excitation spectra .
the dispersed spectra are obtained with fixed excitation laser wavelength and the fluorescence spectrum is analyzed . in excitation spectra , the laser wavelength exciting the molecules to a higher energy state is scanned and the total fluorescence is measured .
information on lower electronic states can be obtained by dispersed spectra , while the excitation spectra reveal parameters ( energy levels , structure information , etc . ) of upper level electronic states .
a conventional experimental setup for obtaining laser - induced fluorescence excitation spectra and dispersed spectra are depicted in figure 1 .
the sample vapor is injected into a vacuum chamber , whose vacuum is maintained by a molecular pump and a mechanical pump , via a supersonic jet . the excitation laser beam ( probe beam )
is provided by a frequency doubled dye laser pumped by a nd : yag laser .
the probe beam intercrosses the sample 10 to 15  mm downstream of the supersonic jet nozzle ; when free radicals are studied , a pump laser beam propagates across the sample close to the nozzle and generates the free radicals .
fluorescence of the excited molecules is collected perpendicular to the probe beam by the optics and imaged onto the detection devices . for excitation spectra ,
total fluorescence is detected by photomultiplier tube ( pmt ) and converted into electronic signals , which are received and displayed in real time on an oscilloscope ( synchronized by the triggers from the nd : yag laser ) .
a general - purpose interface bus ( gpib ) card installed in the computer transfers data from the oscilloscope to the computer via the gpib port , and data acquisition and processing are executed by the vi system . at the same time
, the vi controls the dye laser scans by sending trigger pulses to the laser controller via the parallel port of the computer .
the acquisition and control process for dispersed spectra are similar to those for excitation spectra . for dispersed spectra ,
the vi system communicates with and controls the scan of the monochromator via the usb port in the monochromator .
in addition , the vi system also communicates with the vacuum gauge via an rs232 port , and the system displays the in situ values of the vacuum .
the vi system also builds up an interface to communicate with the operating program of the individual instruments ( e.g. , the dye laser and the molecular pump ) ; specific operating programs are usually included with the instruments at the time of purchase .

 figure 2 shows the flow chart of the vi system that performs the lif experiments and data processing tasks .
it incorporates several subvis , each one of which carries out a specific function , including data acquisition and display , data processing , as well as generating and saving the results .  
there are three icons , excitation spectrum , dispersed spectrum , and data processing , on the front page of the virtual instrument system , each is dedicated to a specific action or experimental technique . when an icon is selected , the corresponding subvi program will be activated , and it will open the panel where specific tasks can be executed . for programming , an event structure for incident response is used to call the corresponding subvi .
when the subvi starts , the event structure waits until the precompiled program code is executed and the task is finished , then the event structure returns to wait for another event .   to fulfill the requirements of lif excitation spectra ,
the excitation spectrum subvi consists of fluorescence data acquisition , data display and saving , vacuum monitoring , communication with other application modules , and synchronization of laser - wavelength - scan subvis .   in the experiment ,
the tds3032b oscilloscope supports the gpib interface communication with a computer where the interface is an eight - bit parallel bus that provides high - speed data transfer and multiple - instrument control .
we invoked the gpib write function from the labview instrument i / o function to set the gpib address of the oscilloscope and specify the data parameters to be transferred from the oscilloscope , including the portion of the waveform that users want to transfer , the waveform source , data format , and the number of bytes per data point .
we then invoked the gpib read function and wired the corresponding gpib address of the oscilloscope to the gpib read , and also specified the number of bytes the function read from the oscilloscope .
finally , we called the function nodes math pattern , string subset , decimate 1d array , and join numbers to receive waveform data from the oscilloscope and display it in situ in the front panel waveform graph .
data from different oscilloscope channels ( ch1 , ch2 , or dual chs ) can be acquired by wiring selected terminals ( in a combo box ) to the case structure prior to data acquisition . for the end - user , the above process is easily achieved by choosing the channel from the pull - down list on the front panel ( figure 4 ) and clicking on the acquire button .
the left one performs the analysis and processing of the fluorescence data obtained from the tds3032b oscilloscope at a particular laser wavelength .
four cursors were created , and property nodes were set to acquire the cursor 's horizontal axis positions on a waveform graph and to define the gate width .
we then used the array subset to return two portions of fluorescence data between the two sets of cursors . on the front panel
, users can directly drag the cursors to where they want , or they can simply set positions in the cursor legend to choose the gate on the graph .
integrals of data in both gates are computed separately using the 1d numeric integration.vi , in which the integration method is the trapezoidal rule .
the difference between the two gate integrals is calculated for the purpose of background subtraction .
finally , the fluorescence intensity and the current wavelength are sent to the xy graph for real - time spectroscopy display .
the front panel of the fluorescence excitation spectrum is shown in figure 4 , in which choose gates is shown in the upper portion and spectroscopy display in the lower part .   
as soon as the data processing is complete , a ttl trigger is sent to step the dye laser to the next wavelength .
ttl subvi , shown in the right sequential diagram in the case structure of figure 3 , calls the outport.vi in port i / o , which specifies a parallel port address for sending the trigger pulse and also defines the pulse width .
a bnc cable is made with the inner core wired to the specified ttl pulse pin of the computer parallel port , and the outside shield wired to the ground pin .
thus , a ttl signal can be sent to the dye laser control unit from the computer parallel port when a data collection cycle is completed and the laser is stepped to the next scan wavelength . the settings for start position , end position , and step size of the laser wavelength scan must be identical in our vi program and in the laser control software .
if the current position is smaller than the end position , the comparison function will return a false value to initiate the program ttl.vi which will move the laser to the next wavelength to start a new data acquisition and processing cycle .
if the current position is greater than or equal to the end position , the true case gives a message dialog box indicating that data acquisition ( daq ) is finished .
subsequently , data can be saved by employing the write to spreadsheet file and the build array and transpose 2d array nodes .
this task is completed by clicking on the save button on the front panel and the fluorescence data will be saved according to the file name entered . the data acquisition process can be stopped at anytime by clicking the acquire button again ( in case the user wants to interrupt the experiment ) .
an abort function is also programmed using the stop node in the application control function , so users can terminate the whole vi system by clicking on the abort - execution button in the upper - right corner ( shown in figure 4 ) . for convenience ,
the help function of the program can be shown by clicking on the help button on the front panel .
the program for the real - time display of the vacuum gauge is shown in the lower - left parallel loop of figure 3 . to communicate with the vacuum gauge ,
parameters such as visa resource name , baud rate , data bits , and parity in the visa configure serial port are set according to the vacuum gauge communication protocol [ 19 , 20 ] .
the visa write function sends command characters to the vacuum gauge and receives vacuum values in return .
the nodes string to byte array , string to single precision float , and index array transform the vacuum data into the required format and display the values on the front panel indicator . for convenience ,
the vi system was designed to interface with the other applications that already exist for the individual instruments . by clicking on the access - other - programs button on the excitation spectrum panel in figure 4
the browse boolean command is wired to the upper case structure , which includes a file dialog express vi .
the application can be selected when the file path is displayed on the front panel .
when the user confirms the file selection , the application program is activated via the system exec.vi shown in the lower case structure in figure 5 .
a change in the dispersed spectrum program is the addition of synchronizing fluorescence daq with a monochromator wavelength scan instead of a dye laser scan .
the 300i monochromator used in our dispersed lif experiment was an omni-3008 spectrometer ( beijing zolix instrument company ) .
communication between the omni-3008 spectrometer and the computer is achieved via a usb port on the omni-3008 spectrometer .
the omni-3008 spectrometer is operated by a zolixomnispec activex ( omnispec ; provided by the manufacturer ) . in the labview vi system
, we invoke an activex container and created a zolixomnispec activex object ; an automation open in the activex functions palette is called , which returns an automation reference number and allows the specific functions programmed in the zolixomnispec object to be accessed .
the omni-3008 spectrometer usb serials string is found by calling the searchzolixusbdevice and getzolixusbserial invoke methods , and then the string is wired to the usbserials property node .
the invoke commands ( open and connect ) are used for setting the connection status of the monochromator , and the status is displayed by the boolean indicator on the front panel .
the initial states of the omni-3008 monochromator components , such as the exit port , grating and current wavelength , are obtained by employing the getexitport , getcurrentgrating , and currentwave methods , respectively .
this step involves the user 's input of the experimental parameters by selecting from the pull - down menu on the front panel .
the setexitport and setcurrentgrating in the case structure return to the default state ( which is set as do nothing ) as soon as the commands are given to avoid sending looped control commands to the omnispec .
the prompt user for input function in the dialog and user interface is called , and it prompts the user to input a specific wavelength , which is subsequently sent to the wavelength terminal in the movetowave method . after using the movetowave method ,
a similar comparison case structure is applied here to step the monochromator scan wavelength as the one used in the excitation lif spectrum program .
identically to the excitation experiment , the start position , end position , step size of the monochromator scan , and the gate for fluorescence signal integration will await user input on front panel . executed from the front panel ,
data acquisition will begin , and the real - time spectrum will be displayed in the xy graph . using the save icon on the front panel ,
an automation close is programmed in the vi system to disconnect the usb interface to the omni-3008 and release the system resources when the dispersed spectrum subvi is closed .
the data processing subvi ( figure 8) can conveniently redisplay and analyze previously saved data [ 21 , 22 ] .
this subvi can automatically accomplish wavelength calibration , unit conversion , and peak and band - width reading of the spectrum curve .
users can also inspect each dataset using cursor move , data refresh , figure enlarge and diminish , and graph recover tools . in the vi programming , a file dialog function will prompt the user to upload a data file . the read from spreadsheet file.vi and index array function will convert the wavelength and intensity data to x and y arrays , respectively .
wavelength unit conversion is realized by the case structures , and then the processed data will be sent to the xy graph for display and saving by write to spreadsheet file.vi .
the vi system reported here has been used to take fluorescence spectra of molecules and free radicals in our laboratory .
the excitation spectrum of a - methylnaphthalene is shown in figure 4 ( in wavelength , nm ) and figure 8 ( in wavenumber , cm ) .
dual chs were used for data acquisition , one for studying the fluorescence signal and another for monitoring the laser power fluctuation using a photodiode . high resolution and signal - to - noise ratios
the obtained excitation spectrum was consistent with previously reported results [ 23 , 24 ] with spectral origin at 31767  cm ( 314.80  nm ) .
the dispersed fluorescence spectrum of a - methylnaphthalene , shown in figure 6 , was obtained by exciting the most prominent band at the wavelength of 310.70  nm .
dispersed spectra utilizing excitation of other bands ( 306.50  nm and 314.80  nm ) were also obtained .
the vi system integrates a variety of laboratory equipment and controls the experiment data acquisition process using a single computer via gpib , usb , rs232 , and parallel ports .
data display , saving , and analysis can also be accomplished using the reported vi system .
furthermore , the vi system can monitor running of individual instruments and changing of vacuum and laser power in real time .
therefore , the users can timely judge , analyze , and process on - site information , and high resolution and signal - to - noise ratios of molecular or free radicals can be easily achieved in the spectra .
our approach has been to develop the vi system for general use in lif experiments .
the instruments used in this paper , such as the nd : yag laser ( sureliteiii , continuum ) , dye laser ( narrowscan , radiant dye ) , digital delay generator ( dg535 , stanford ) , oscilloscope ( tds3032b , tektronix ) , and pmt ( cr110 , hamamatsu ) are commonly used in many laboratories . when different instruments are to be used in an experiment , the users simply replace the communication method according to the communication protocol for the corresponding replacement instrument or change the file path in the access - other - program subvi by following the instructions provided in the vi .
the vi system has user - friendly front panel windows and is easy to operate .
help and instructions implanted in the program will assist the user in becoming familiar with each application step .  
the vi system was tested extensively and applied in the determination of the excitation spectrum and the dispersed spectrum of -methylnaphthalene with satisfactory experimental results .
the vi system promotes and expands the capabilities of individual instruments , and it is an intelligent improving for existing hardware sources of laboratory .
lots of complicated functional operations are merely finished at the click of button on the vi system .
the vi system not only avoids the manual error during the separate instrument control , data acquisition , and data processing , but ensures the accuracy and repeatability of experimental data and improves the efficiency of experiment .
furthermore , the vi system has a wide expandability of functions , minor modifications can be applied to a different lif setups based on automatic measurement and control .
 we report the design and operation of a virtual instrument ( vi ) system based on labview 2009 for laser - induced fluorescence experiments . 
 this system achieves synchronous control of equipment and acquisition of real - time fluorescence data communicating with a single computer via gpib , usb , rs232 , and parallel ports . 
 the reported vi system can also accomplish data display , saving , and analysis , and printing the results . 
 the vi system performs sequences of operations automatically , and this system has been successfully applied to obtain the excitation and dispersion spectra of -methylnaphthalene . 
 the reported vi system opens up new possibilities for researchers and increases the efficiency and precision of experiments . 
 the design and operation of the vi system are described in detail in this paper , and the advantages that this system can provide are highlighted . 
1. Introduction
2. Configuration of System Hardware
3. Programming
4. Application
5. Conclusions