IR beam break with time data logger - energy efficient

Hello,

I am still very new and learning all of this. I am trying to make an energy efficient IR emitter/sensor device that will record the time of an IR beam break. It needs to run for several months. I am considering using the adafruit data logger shield to keep track of time and record the data on an SD card. I know arduino can be power hungry so I was considering using either an Arduino Pro Mini 328 or ATtiny85, but I don't really know if these are appropriate, capable, energy efficient, etc. I will appreciate any and all input about what materials are best here. Thanks!

Running any electronics for several moths calls for lots of thinking about the power supplying. Battories selfdischarge…. Maybe solar power can be used? What about climate issues, low/highy temperatures etc.?
I beleive this is probably more important than this or that controller even if controller thurst for current is as well important.

What are you detecting with the beam break device?
Is it something momentary like a car passing or is it something quite static like a letter lying in a mailbox?
You may be able to design the circuit in such a way that a detection wakes the Arduino from sleep mode, which in turn reads a real time clock module, logs the event to an external eeprom, then goes back to sleep. A ‘barebones’ Arduino can have a very low power consumption. The detector part has to be designed or selected carefully for optimal power consumption.

"Beam break" systems usually use a modulated beam in order not to be confounded by ambient - generally steady - light or IR radiation.

This is an excellent opportunity to use a beam with a very low duty cycle to save power which will be a major consideration where you are using a beam over some distance. Your receiver can be AC coupled and you can use synchronous sampling so that it only looks for the beam during the short interval that it is turned on.

Why not start by defining the problem with specifications including a flow chart and schematic. List your resources. Define what is the expected outcome. Purchase the Arduino cookbook and read it, this will give you some basics. also use the online tutorials and videos available, there are many good ones on this web site. At this point you will be able to define the problem and may have already solved it.